Broadened glass transition in a liquid-crystalline polymer studied by thermally stimulated currents,AC dielectric,dynamic mechanical,and DSC methods

Thermally stimulated currents (TSC), a. c. dielectric, dynamic mechanical (DMTA), and differential scanning calorimetry (DSC) methods were used to study the glass transition in a thermotropic liquid-crystalline copolyester. All the techniques were consistent in the determination of the main glass transition temperature. Using the high sensitivity of the TSC thermal sampling method, it was shown that cooperative glass transition-like relax-ations occur down to 100°C below the main glass transition. DSC was sensitive only to a broadening of the glass transition to about ca. 30°C, so it was concluded that the thermal sampling method is sensing a very small fraction of cooperatively relaxing species which cannot be detected by DSC. Ac dielectric measurements and DMTA also indicated that the glass transition was broad, but difficulties with overlapping transitions prevented quantitative determination of the breadth of the glass transition. The results suggest that the broad glass transition, in this mostly amorphous LCP, is due to chemical heterogeneity of the copolyester chain. Other evidence indicates that the broadening is not due to the oriented nature of the glassy state. Some discussion is presented concerning how the heterogeneous nature of the LCP glass leads to compensation of the Arrhenius curves obtained by the thermal sampling method. © 1993 John Wiley & Sons, Inc.     

Molecular dynamics and ordering of side chain liquid crystal polymers as studied by paramagnetic resonance

Abstract

Molecular dynamics of side chain liquid crystalline polymers (LCP) and their components were studied using the technique of paramagnetic resonance. A cigar shape spin probe (COL) and a nearly spherical spin probe (TPL) were used to study the motions and order of the LCPs. Computer simulations of the observed spectra were performed. Both rotational correlation times and order parameters were extracted from these simulations. We found that LCPs containing 30 per cent and 50 per cent of mesogenic side chains had about the same viscosity as indicated by nearly equal tumbling times at the same temperature. In addition, the LCPs motion is considerably slower than that of the monomeric liquid crystal indicating that the spacer couples the motions of the side chains to those of the main chain. Rotations about axes perpendicular to the side chain are slowed more than rotations about an axis parallel to the side chain. DSC measurements were employed to study the phase transitions. The 30 and 50 per cent LCPs displayed first order NS_A transitions, but the 50 per cent LCPs transition was much weaker, in agreement with McMillan's theory which predicts a first order transition for T _NS/T _NI>0.87 (observed ratios are 0.98, 0.90 and 0.86 for 30, 50 and 100 per cent LCPs, respectively). The 30 per cent LCP has a very short nematic range so that the nematic order, which is not saturated at the NS transition, can couple with the smectic order. This was indicated by a sharp change in slope of the order parameter versus temperature plot as the smectic is entered. The LCPs studied formed a highly ordered glass when cooled in a 1 T field. If one could find a LCP with similar ordering properties whose glass temperature is well above room temperature, then one would have a useful binder for the manufacture of haze-free polymer dispersed liquid crystal displays.     

Cooperative motions in PVC studied by thermally stimulated currents: Comparison with A.C. dielectric derivative analysis

The thermally stimulated current–thermal sampling (TSC–TS) technique was used to study the broadened glass transition in conventional “atactic” poly(vinyl chloride), PVC. The activated parameters obtained from the TSC–TS data, mainly the apparent activation energy (E_a), characterize the breadth of glass transitions in a very sensitive way. These results are compared with those values of E_a obtained from the literature, using a recently proposed method of analyzing a.c. dielectric constants and their derivatives, over the temperature range of −100–130°C. Both techniques detect weak cooperative glass transition-like relaxations well below the main glass transition of ca. 80°C. As is the case with “atactic” PMMA, the data suggest that compositional heterogeneity related to a small fraction of predominantly isotactic sequences contribute to the broad glass transition extending ca. 60°C below the main glass transition in atactic PVC. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 913–918, 1998     

Thermally stimulated current studies of thermotropic polyesters with polymethylene spacers (I): The glass transition and relaxation behavior of anisotropic and isotropic glasses

Thermally stimulated current (TSC) and relaxation map analysis (RMA) were used to study the glass transitions and relaxation phenomena of the anisotropic and isotropic glasses for the semiflexible polyesters with various polymethylene spacers (n = 7 ˜ 10). TSC analysis indicated that the glass transition temperatures (T_g) of polymers at around 40 ˜ 50°C exhibited an apparent even-odd behavior not only for the anisotropic glasses but also for the isotropic glasses. The T_g of the isotropic glass for the polymer with even n value was observed to be higher than those for the two neighboring polymers with odd n values. The anisotropic glass for the polymer with even n value had a lower T_g than those for the two neighboring polymers with odd n values. The lower value was attributed to the configuration and orientation effects on the behavior of polarization. RMA revealed that the relaxation modes of the investigated polymers were also influenced by the configuration and orientation effects. The dipolar relaxation of the anisotropic glass for the polymer with odd n value occurred at a higher temperature and had a lower entropy (enthalpy) of activation than that of the isotropic glass for the same polymer due to the orientation effect. However, an inverse relation was found to occur for the polymer with even n value, which came from the trans configuration of the even polymethylene spacers. Finally, the thermokinetic properties evaluated by RMA (e.g., the final state after depolarization) correlated quite well with the results obtained by TSC. © 1995 John Wiley & Sons, Inc.     

STUDIES ON MISCIBILITY AND MORPHOLOGICAL FEATURES OF BLENDS OF LC COPOLYESTER AND PET

The morphology of a special blend system composed of liquid crystalline aromatic random copolyester (LCP) and semiflexible polyester PET over the whole composition range has been studied by means of polarized microscope, density measurement, DSC, FTIR and SEM. Based on the microscopic observation, it is found that under suitable mechanical mixing condition, LCP may be rather homogeneously dispersed in the PET matrix, with the middle composition range of the contents of LCP at 30--70 wt % the anisotropic and isotropic phase segregation appears, while with LCP contents over 80 wt% the blends exhibit wholly anisotropie. The DSC thermographs of the melt-pressed and quenched films show single T_(?), T_(cc) and T_m. T_(?) increases with increasing content of LCP and ap, proaches to the T_(?) of pure LCP. The experimental results indicate that the two components of this blend system are miscible, there exist some specific interactions between them.     

The rigid amorphous fraction in semicrystalline macromolecules

The first experimental evidence of the existence of the rigid amorphous fraction (RAF) was reported by Menczel and Wunderlich for several semicrystalline polymers. It was observed that the hysteresis peak at the glass transition was absent when these polymers were heated much faster than they had previously been cooled. In the glass transition behavior of poly(ethylene terephthalate) (PET), the hysteresis peak gradually disappeared as the crystallinity increased. At the same time, it was noted that the ΔC _p of higher crystallinity PET samples was much smaller than could be expected on the basis of the crystallinity calculated from the heat of fusion. It was also observed that this behavior was not unique to PET only, but is characteristic of most semicrystalline polymers: the sum of the crystallinity calculated from the heat of fusion and the amorphous content calculated from the ΔC _p at the glass transition is much less than 100% (a typical difference is ~20–30%). This 20–30% difference was attributed to the existence of the “RAF”. The presence of the RAF also affected the unfreezing behavior of the “mobile (or traditional) amorphous fraction.” As a consequence, the phenomenon of the enthalpy relaxation diminished with increasing rigid amorphous content. It was suggested that the disappearance of the enthalpy relaxation was caused by the disappearance or drastic decrease of the time dependence of the glass transition. To check the validity of this suggestion, the glass transition had to be also measured on cooling in order to overlay it on the DSC curves measured on heating. However, before this overlaying work could be accomplished, the exact temperatures on cooling had to be determined since the temperature of the DSC instruments that time could not be calibrated on cooling using the usual low molecular weight standards due to the common phenomenon of supercooling. Therefore, a temperature calibration method needed to be developed for cooling DSC experiments utilizing high purity liquid crystals using the isotropic → nematic, the isotropic → cholesteric, and other liquid crystal → liquid crystal transitions. After the cooling calibration was accomplished, the cooling glass transition experiments indicated that the glass transition in semicrystalline polymers is not completely time independent, because its width depends on the ramp rate. However, it was shown that the time dependence is drastically reduced, and the midpoint of the glass transition seems to be constant which can explain the absence of the enthalpy relaxation. The work presented here has led to a number of studies showing the universality of the rigid amorphous phase for semicrystalline polymers as well as an ASTM standard for DSC cooling calibration.     

A.C. dielectric and TSC studies of constrained amorphous motions in flexible polymers including poly(oxymethylene) and miscible blends

Poly(OxyMethylene) (POM) and its miscible blends were studied by multifrequency A.C. dielectric and thermally stimulated currents (TSC). The blends contained small amounts of either poly(vinyl phenol), which is a high glass transition (T_g) diluent, or a styrene-co-hydroxy styrene oligomeric low T_g diluent. The variation of the 10°C “β” transition with blend composition proves that it is the glass transition, and that the −70°C “γ” transition is a local motion. Dielectrically the β transition is very weak in pure POM even in fast-quenched samples. The TSC thermal sampling method also detected two cooperative transitions, γ and β, in POM and its blends, and was used to directly resolve the γ transition into low and high activation energy components. If one considers the contribution of exclusion of the diluents from the crystal lamellae, it is shown that the blends behave like typical amorphous blends as a function of concentration. The effect of crystals on amorphous motions is examined in light of comparison with van Krevelen's³⁷ predictions of an “amorphous” T_g, and the transitions in POM are contrasted with those for other semicrystalline polymers. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2121–2132, 1997     

Equilibrium phase diagram of polystyrene and 8CB

The experimental equilibrium phase diagram of a mixture of linear polystyrene of molecular weight M_w = 44,000 g/mol and 4‐cyano‐4′‐n‐octyl‐biphenyl (8CB) is established. The three transitions smectic A‐nematic, nematic‐isotropic, and isotropic‐isotropic are observed. The first two are observed both by optical microscopy and differential scanning calorimetry (DSC) while the isotropic‐isotropic transition could be seen only via optical microscopy. Two series of samples with the same compositions were independently prepared and yielded consistent results both by microscopy and DSC. Measurements of sample compositions with less than 50 weight % of 8CB were influenced by the vicinity of the glass transition temperature (T_g) of the polymer in the mixture. This quantity is also determined by DSC as a function of composition. A single T_g is observed, which decreases with composition of the LC. Other thermodynamic quantities such as the enthalpy variations of LC in the nematic‐isotropic transition and the fraction of LC contained in the droplets are also considered. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1841–1848, 1999     

Gas transport properties of liquid crystalline poly (ethylene terephthalate-co-p-oxybenzoate)

Gas transport properties are reported for a series of films prepared from thermotropic poly (ethylene terephthalate-co-p-oxybenzoate), or PET/PHB, having compositions of 60 and 80 mol % PHB. The mesomorphic and crystalline morphology of the copolyester films was examined by cross-polarized light microscopy, differential scanning calorimetry (DSC), and x-ray diffraction. Melt-processed films of both compositions appeared to exhibit an entirely anisotropic morphology with low levels of conventional crystallinity. Solution-cast films prepared from the 60 mol % material were found to contain a large fraction of isotropic regions, which become ordered upon annealing above the glass transition. Permeability measurements were made for He, H₂, O₂, N₂, and CO₂ at 35°C and the diffusivities were computed from time-lag data. The largely anisotropic films exhibit good barrier properties resulting from very low solubility coefficients. The partially isotropic 60 mol % films show much higher permeability coefficients driven primarily by increased solubility coefficients, while diffusivity is affected to a lesser extent. These results appear to contrast with what is observed in semicrystalline systems where increased crystalline order results in more dramatic reductions in penetrant mobility.     

The influence of annealing on thermal transitions in a nematic copolyester

Thermal transitions of a glassy, main chain, liquid crystalline, random copolyester, HIQ‐40, have been characterized. HIQ‐40 is made from 40 mol percent p‐hydroxybenzoic acid (HBA) and 30 mol % each of p‐hydroquinone (HQ) and isophthalic acid (IA). This polymer is soluble in organic solvents, permitting the preparation of thin, solution‐cast films that are in a glassy, metastable, optically isotropic state. On first heating of an isotropic HIQ‐40 film in a calorimeter, one glass transition is observed at low temperature (approximately 42°C), and is ascribed to the glass/rubber transition of the isotropic polymer. A cold crystallization exotherm centered near 150°C is observed. This is associated with the development of low levels of crystalline order. A broad melting endotherm is centered at about 310°C; this endotherm marks the melting of crystallites and the transformation to a nematic fluid. A nematic to isotropic transition was not observed by calorimetry. After quenching from the nematic melt, a T_g is observed in the range of 110–115°C and is associated with the glass/rubber transition of the nematically ordered polymer. Annealing optically isotropic films at temperatures above the isotropic glass transition results in the systematic development of axial order. In these annealed samples, T_g increases rapidly until it is near the annealing temperature, then T_g increases more slowly at longer annealing times. In as‐cast films annealed at 120–135°C, the light intensity transmitted through a sample held between crossed polarizers in an optical microscope (a qualitative measure of birefringence and, in turn, axial order) initially increases rapidly and uniformly throughout the sample and, at longer annealing times, approaches asymptotic values that are higher at higher annealing temperatures. The increase in transmitted intensity is ascribed to the development of axial order. The uniform increase in transmitted intensity suggests that ordering occurs by a rather global process and not via a nucleation and growth mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 505–522, 1999     

Detection of obscured glass transitions by QiDSC

Determination of compatibility in the amorphous phase for a two component blend is usually accomplished by analyzing for whether one notes one or two glass transitions. This can be complicated when one of the components is semicrystalline and its melting peak obscures the second glass transition. Quasi-isothermal differential scanning calorimetry (QiDSC) can be used to detect an obscured glass transition by allowing the semicrystalline component to melt and relax revealing the underlying glass transition of the other component. QiDSC is accomplished by performing a modulated temperature DSC experiment at a particular temperature and step ramping through the transitions of interest. For this study two systems are investigated. The first system is a model system based on a blend of polystyrene (PS) and a copolymer of vinylidene fluoride and hexafluoropropylene, P(VF₂/HFP). The glass transition for the PS occurs at the same temperature as the melting point for the fluoro-copolymer. The second system is a fluoro-copolymer/acrylic dried latex. In both cases the hidden glass transition can be noted in the reversing heat capacity of the QiDSC analysis.     

Thermophysical properties of azomethine ether main-chain liquid crystalline polymers at pressures to 200 MPa

This article reports on an experimental investigation of the equation of state and the transition behavior of main-chain thermotropic liquid crystalline polymers over a wide temperature range, and at pressures to 200 MPa. The materials studied were a series of azomethine ether polymers. A varying number n (= 4, 7, 8, 9, 10 and 11) of methylene spacer units in the backbone provided systematic variation of the structure. Experimental techniques used included high-pressure dilatometry (PVT measurements) to 200 MPa, high-pressure differential thermal analysis, also to 200 MPa, and conventional (atmospheric-pressure) differential scanning calorimetry (DSC). The equation of state of the materials can be well represented by the Tait equation in distinct regions, separated by a glass transition, T_g(P), a first-order transition to a nematic state, T_k-n(P), and a first-order transition to an isotropic melt state T_c(P). The atmospheric pressure values of T_k-n and T_c decreased with increasing number of spacer units and showed a clear odd-even effect. T_g and T_k-n both increased with pressure. The pressure dependence of T_c could not be observed due to the onset of degradation in the same temperature region. On isobaric cooling at 3°C/min, the crystallization from the nematic state occurred a few tens of degrees below T_k-n. This supercooling was independent of pressure for some materials, while for others it increased with increasing pressure. The values of the enthalpy and entropy associated with the first-order transition into the nematic state were lower than those of typical isotropic polymers at their melting transitions. The transition enthalpy did not have any systematic variation with increasing number of spacer units. Values of the transition enthalpy calculated from the Ciapeyron equation did not always agree with the values measured by DSC. This may be due to the two-phase nature of the low-temperature state. At the transition to the isotropic state, the transition enthalpy at P = 0 decreased with n and showed an odd-even effect. © 1994 John Wiley & Sons, Inc.     

Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

Thermomechanical properties of polymers highly depend on their glass transition temperature (T _g). Differential scanning calorimetry (DSC) is commonly used to measure T _g of polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuring T _g in such type of polymers. The changes in specific heat capacity (Δc _p) at T _g were accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc _p from flexible polymer (0.28 J g^?1 K^?1 for polystyrene) to rigid CPs (10^?3 J g^?1 K^?1 for a naphthalene diimide‐based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the Δc _p at T _g by a factor of 10, confirming that backbone rigidity reduces the Δc _p. Additionally, an increase in the crystalline fraction of the CPs further reduces Δc _p. We conclude that the difficulties of determining T _g for CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D‐A CPs with weak transition signals. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1635–1644     

Phase transitions in mesophase macromolecules. VI. The transitions in poly(oxy-2,2′-dimethylazoxybenzene-4,4′-diyloxydodecanedioyl)

The transitions of poly(oxy-2,2′-dimethylazoxybenzene-4,4′-diyloxydodecanedioyl) (PDAD) have been analyzed by differential scanning calorimetry, optical microscopy, and light scattering. The mesophase glass devitrifies at 288 K [ΔC_p = 220 J/K mol]. Crystallization from the liquid mesophase can be described between 322 and 362 K by an Avrami expression with an exponent between 3 and 4. Results of light scattering and optical microscopy are in accord with a spherulitic morphology grown after athermal nucleation. Melting of the semicrystalline samples (crystallinity up to 58%) occurs at about 391 K. The heat of fusion of the completely crystalline sample is calculated to be only 13.55 kJ/mol. The mesophase to isotropic phase transition occurs at 418 K with a heat of transition of 4.1 kJ/mol. A general discussion of these transitions is given.     

Interrelation between side chain crystallization and dynamic glass transitions in higher poly(n-alkyl methacrylates)

Calorimetric and dielectric results for crystallizable poly(n-alkyl methacrylates) (PnAMA) with C=12, 16 and 18 alkyl carbons per side chain are presented. Degree of crystallization D_cal and melting peak temperature T_M are estimated from conventional DSC measurements. For poly(n-hexadecyl methacrylate) (C=16) the influence of isothermal crystallization is studied by DSC as well as TMDSC. Changes in dielectric relaxation strength Δε and α peak shape during crystallization are investigated. Effects of side chain crystallization on the complex dynamics of PnAMA are discussed. The results are related to the relaxation behavior of lower nanophase-separated PnAMA with two co-existing glass transitions, the conventional glass transition (a or α) and the polyethylene-like glass transition (α_PE) within alkyl nanodomains formed by aggregated alkyl rests. It is shown that amorphous as well as semicrystalline PnAMA can be understood as nanophase-separated polymers with alkyl nanodomains having a typical dimension of 1-2 nm. The results are compared with the predictions of simple morphological pictures for side chain polymers. X-ray scattering data for the amorphous and semicrystalline PnAMA are included in the discussion. Common aspects of nanophase-separated systems in both states as well as differences caused by crystallization are discussed. Indications for the existence of rigid amorphous regions are compiled. Different approaches to explain a similar increase of T_g(α_PE)—the glass temperature of the amorphous alkyl nanodomains—and T_M—the melting temperature of crystalline alkyl nanodomains—with side chain length are considered. Pros and cons of both approaches, based on increasing order within the alkyl nanodomains and confinement effects in nanophase-separated systems, are discussed. Main trends concerning crystallization and cooperative dynamics are compared with those in other systems with self-assembled nanometer confinements like microphase-separated blockcopolymers or semicrystalline main chain polymers.     

Broadening of the glass transition in blends of poly(aryl ether ketones) and a poly(ether imide) as studied by thermally stimulated currents

Blends of poly(aryl ether ketones) (PAEKs) and an amorphous poly(ether imide) (PEI) were used as model systems to study the broadening of the glass transition due to crystallization and the resulting depletion of PAEK from the amorphous phase. Two different PAEKs were studied, which are completely miscible with PEI in the amorphous state; poly(aryl ether ether ketone) (PEEK) and a slower crystallizing poly(aryl ether ketone ketone)(PEKK). Relatively rapid crystallization conditions were chosen in order to trap a significant fraction of PEI between the PAEK crystal lamellae or between bundles of lamellae. The broad glass transitions are apparently a result of the nonuniform nature of this process. The breadth of the glass transition was quantified by thermally stimulated currents (TSC) applied in the thermal sampling (TS) mode. The results compared favorably with DSC data. The magnitude of the apparent activation energy obtained by the TS method allows one to assign the relaxations as cooperative (glass transition-like) or non-cooperative and to define the limits of the glass transition with a higher degree of precision than other techniques. Cooperative relaxations can be resolved with this technique, even if they are only a small fraction of the overall relaxing species at a given temperature. In some cases the glass transition region was found to broaden to ca. 60°C after crystallization. © 1993 John Wiley & Sons, Inc.     

How did we find the rigid amorphous phase?

The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich [1]: when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers [poly(ethylene terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] [2], the absence of the hysteresis peak at the lower temperature glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled. Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity was confirmed [1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC _p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses of polymers. When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC _p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min⁻¹ rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF, Nylon 66 and polycaprolactone [1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate [3] were added to this list.     

Thermal analysis of Vectran® fibers and films

Vectra^® liquid crystalline polymers (LCP's) were introduced as commercial products in the mid-1980's. The first of these (Vectra A130) was a wholly aromatic thermotropic copolyester ofp-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. Vectra A130 is a thermotropic LCP that can be melt spun into filaments that on heat treatment are characterized by high strength and high modulus. Vectra resin can also be extruded into films. In the fiber or film form this material is commercially known as Vectran^®. Heat treatment enhances the tensile strength of Vectran fiber variants. Because of this, the elucidation of the physical transformations taking place in the internal structure of the material during heating has always been an important subject. Several thermal techniques are used to indicate clearly that what is observed as a glass transition is unlike the conventional glass transition in typical semicrystalline polymers. There is also an indication of the presence of multiple states of mesophase aggregation that collapse into a single state when taken to high enough temperatures.     

The amount of immobilized polymer in PMMA SiO2 nanocomposites determined from calorimetric data

For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals are melting first and simultaneously the RAF devitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as the RAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do not melt or undergo other phase transitions altering the polymer-nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO₂ nanocomposites was shown on the basis of heat capacity measurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO₂ nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transition was found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax.