Thermally stimulated current and DSC studies of the broadened glass transition in liquid crystalline polymers

The high sensitivity of the thermally stimulated current, thermal sampling (TS) method is emphasized in a study of the breadth of the glass transition in several liquid-crystalline polymers (LCPs). Differential scanning calorimetry (DSC) was performed on all samples to further quantify the glass transition regions. For “random” copolyester LCPs with widely varying degrees of crystallinity, including highly amorphous samples, very broad glass tran-sition regions were observed. One semicrystalline alternating copolyester and a series of semicrystalline azomethine LCPs were studied as examples of structurally regular polymers. These exhibited relatively sharp glass transitions more comparable to ordinary isotropic amorphous or semicrystalline polymers. The broad glass transitions in the random copolyesters are attributed to structural heterogeneity of the chains. In one example of a moderate-crystallinity random copolyester LCP (Vectra), glass transitions ranging up to ca. 150°C in breadth were determined by the thermal sampling (TS) method and DSC. In other lower crystallinity copolyester LCPs, the main glass transition temperature as determined by DSC was comparable to that determined by TSC although cooperative relaxations of a minor fraction of the overall relaxing species were detected well below the main T_g, by the TS method and not by DSC. Rapid quenches from the isotropic melt to an isotropic glass were possible with one LCP. The anisotropic and isotropic glassy states for this LCP were found to have the same breadth of the glass transition as was determined by the TS method, although TSC and DSC show that T_g is shifted downward by ca. 15°C in the anisotropic glass as compared to the isotropic glass. © 1993 John Wiley & Sons, Inc.     

Synthesis and characterization of mesogen‐jacketed liquid‐crystal polymers based on 2,5‐bis(4′‐alkoxyphenyl)styrene

A series of novel mesogen‐jacketed liquid‐crystal polymers, poly[2,5‐bis(4′‐alkoxyphenyl)‐styrene] (P‐n, n = 1–11), were prepared via free‐radical polymerization of newly synthesized monomers, 2,5‐bis(4′‐alkoxyphenyl)styrene (M‐n, n = 1–11). The influence of the alkoxy tail length on the liquid‐crystalline behaviors of the monomers and the polymers was investigated with differential scanning calorimetry (DSC), thermogravimetry, polarized optical microscopy (POM), and wide‐angle X‐ray diffraction (WAXD). The monomers with n = 1–4, 9, and 11 were monotropic nematic liquid crystals. All other monomers exhibited enantiotropic nematic properties. Their melting points (T_m's) decreased first as n increased to 6, after which T_m increased slightly at longer spacer lengths. The isotropic–nematic transition temperatures decreased regularly with increasing n values in an odd–even way. The glass‐transition temperatures (T_g's) of the polymers first decreased as the tail lengths increased and then leveled off when n ≥ 7. All polymers were thermally stable and entered the mesophase at a temperature above T_g. Upon further heating, no mesophase‐to‐isotropic melt transition was observed before the polymers decomposed. WAXD studies indicated that an irreversible order–order transition for the polymers with short tails (n ≤ 5) and a reversible order–order transition for those with elongated tails (n ≥ 6) occurred at a temperature much higher than T_g. However, such a transition could not be identified by POM and could be detected by DSC only on heating scans for the polymers with long tails (n ≥ 7). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1454–1464, 2003     

Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Difunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH₂)_n(n), and M is a mesogen—in this case 4,4′-dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high T_g can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final T_g—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid-state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with T_g = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (T_i), a poorly organized polymer was obtained with a T_g of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below T_i, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above T_i. There was little effect of polymerization temperature on T_g when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.     

Synthesis and thermal transitions of a soluble,main chain,nematic liquid crystalline polymer exhibiting a kinetically trapped,disordered structure

An aromatic copolyester composed of 25 mol % phenyl hydroquinone, 10 mol % isophthalic acid, 40 mol % chloroterephthalic acid, and 25 mol % t-butyl hydroquinone (PICT) has been synthesized. This amorphous, glassy polymer is soluble in common organic solvents such as methylene chloride. Thin, solution-cast films may be prepared which are in a metastable, vitrified, optically isotropic state. On first heating of an isotropic film at 20°C/min in a calorimeter, one glass transition is observed at low temperature (approximately 49°C) and is ascribed to the glass/rubber transition of the metastable, isotropic polymer. This thermal event is followed by a small exotherm due to the development of order during the scan, which results in a second T_g at approximately 125°C. This T_g is associated with the glass/rubber transition of the ordered polymer. Nematic order can be developed by thermal annealing. The lower T_g increases toward the upper T_g as annealing time is increased. For an initially isotropic film annealed at 90°C, the increase of the lower T_g with annealing time and the increase in birefringence observed by optical microscopy are governed by similar kinetics. Isotropization occurs in the temperature range of 250–300°C. The nematic polymer is slightly more dense than its isotropic analog. No detectable differences between isotropic and nematic samples were observed in rotating frame proton spin lattice relaxation times. © 1996 John Wiley & Sons, Inc.     

Glass transformation temperatures of polymers of olefin oxides and olefin sulfides

A number of polymers belonging to poly(olefin oxide) and poly(olefin sulfide) series have been prepared and their glass transformation temperatures determined by dilatometry and differential thermal analysis. In the poly(olefin oxide) series, the T_g remained practically unchanged as the length of the pendent alkyl group was increased from methyl to n-hexyl. However, a 20°C decrease in T_g was observed when the pendent group was changed from ethoxymethyl to n-hexoxymethyl. In the poly-(olefin sulfide) series, the T_g value decreased as the pendent alkyl group changes from methyl to ethyl. Replacement of ether oxygen in the polymer main chain by sulfide sulfur increased the T_g value. In some polymers, first-order transitions were observed, but their significance has not been assessed.     

Free volume quantities and viscoelasticity of polymer glasses

We have investigated, in terms of the Cohen-Turnbull theory, a relationship for polycarbonate (PC) glasses between average stress relaxation times, <to, and average free volume sizes, 〈v_f〉, obtained from positron annihilation lifetime spectroscopy. This examination suggests that the minimum free volume required for stress relaxation, v*, decreases with decreasing temperature and that, near the glass transition temperature, only a subset of extremely large free volume elements contributes to the stress relaxation of PC glasses. This suggestion is consistent with the idea that near the glass transition temperature, the viscoelastic response is dominated by large-scale, main-chain motion, whereas at lower temperature it is controlled by local motion. Moreover, comparison with the v* value estimated from gas diffusivity through various PC species at room temperature shows that the required free volume size for stress relaxation in the glass transition region is much larger than that for gas diffusion. Previously we showed that the Doolittle equation fails to correlate viscoelastic relaxation times of polymer glasses with changing temperature; determining the free volume fraction, h, from theoretical analysis of volume recovery data and theory, the Doolittle equation is shown to be valid in PC above 135°C (T_g - 14°C) irrespective of temperature and physical aging times. This result supports the idea suggested in the previous article that, as glassy polymers approach the transition region, viscoelastic properties increasingly tend to be controlled by free volume. © 1996 John Wiley & Sons, Inc.     

Glass Transition Temperature and Value of the Relaxation Time at Tg in Vitreous Polymers

Summary: Many works focused on glassy polymers determine values of glass transition temperature (T_g) and an overview of the literature shows that depending on the method used, values of T_g are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility characterized by relaxation time functions. By using three independent experimental procedures (dielectric, thermally depolarized current and calorimetric), we show that the value of the glass transition and the value of the relaxation time at T_g can be correctly determined. It is also shown that the assumption: τ _(Tg) = 100 s is constant, is not correct. The protocol proposed also allows the determination of the value of the fragility index “m” of the glass forming liquid with a great accuracy.     

Correlation between glass transition temperature and chain structure for randomly crosslinked high polymers

The empirical form for the dependence, T_g(n) ≅ T_g(∞)·(1 + α/n), of the glass transition temperature T_g on the average number n of repeat units between crosslinks, is generalized for randomly crosslinked high polymers. The new form, T_g(n) ≅ T_g(∞) · [1 + c/(n·N_rot)], is based on a correlation study of data for 77 samples of 10 different sets of resins. The fitting parameter α is resolved into composition-dependent N_rot and composition-independent c terms. N_rot summarizes the average number of rotational degrees of freedom per repeat unit, and is estimated in a straightforward manner from the structure and mol fraction of each repeat unit. The value of c is found from data analysis to be 5 ± 2. The results of this work are consistent with expectations based on the entropy theory of glasses, and provide improved understanding and predictive ability for the properties of crosslinked polymers. © 1996 John Wiley & Sons, Inc.     

Preparation of polymer nanoparticles, and the effect of nanoconfinement on glass transition, structural relaxation and crystallization

In this review the preparation methods of polymer nanoparticles from chemical microemulsion polymerization to physical methods such as spray-drying, freeze-drying, freeze-extracting, fast evaporation and spreading evaporation have been summarized. The influence of nanoconfinement on glass transition temperature (T _g) variation from significant or slight decrease, no evident T _g deviation, to even T _g increase, as well as possible explanations of T _g deviations were discussed. The influences of nanoconfinement or entanglement on the other properties such as structural relaxation, crystallization in polymer nanoparticle samples were also reviewed in this article.     

A MONTE CARLO STUDY OF THE GLASS TRANSITION OF POLYMETHYLENE*

By this Monte Carlo simulation we studied the glass transition of polymethylene using themodified bond-fluctuation model combined with considering the rotational-isomeric state model. Theconfigurational properties in the polymethylene (PM) melts, such as the mean length, the mean energy perbond and the mean square radius of gyration were monitored. We found that the chains cannot be in theequilibrium states after a very long time when the temperature of the dense PM chains decreases to 120 K. Asthe melt vitrifies, these quantities gradually become independent of temperature in a narrow range. The glasstransition temperature T_g depends upon the chain length of PM chains, and extrapolation to (CH_2)_∞givesT_g~∞=212 K. The dynamics in the PM melts was also studied. It was found that the diffusion coefficients canbe described by the Vogel-Fulcher law and the Vogel-Fulcher temperature T_0 is 124 K. This method may beused to investigate the glass transition of other real polymer chains.     

Calorimetric relaxation and glass transition in poly(propylene glycols) and its monomer

The glass transition temperature T_g of propylene glycol (PG) and poly(propylene glycols) (PPGs) of molecular weight up to 4000 has been measured by differential scanning calorimetry, and the activation energy and change in heat capacity ΔC_p have been determined in the glass transition range. The activation energy increases with an increase in the molecular weight of the polymer, and ΔC_p measured at a fixed heating rate decreases. The increase in T_g with molecular weight is remarkably more rapid for poly(propylene glycols) than for other polymers, and a limiting value of T_g is reached for a chain containing 20 monomer units. These results are discussed in terms of the Fox-Flory and the entropy theories. The calorimetric relaxation times are comparable with the extrapolated dielectric relaxation times. The initial increase of ΔC_p from PG to PPG 200 is attributed to the decrease of H-bonding sites from 12 in 3 monomers to 4 on polymerization to PPG 200 and further decrease with increase in molecular weight to an increasingly large amplitude of the β-process at T < T_g.     

Boson peak and fast relaxation process near the glass transition in polystyrene

Atactic polystyrene, both side group and main chain deuterated, was investigated by inelastic neutron scattering in a wide temperature range around the glass transition from 2 to 450 K. In the glass the Boson peak position is only very weakly influenced by the deuteration of the phenyl group. In the neighborhood of the glass transition temperatureT _g we find a fast relaxation process similar to other glasses. The onset of the fast relaxation in polystyrene, however, is observed already at temperaturesT _g — 200 K. Results from partially deuterated polystyrene suggest a change of the phenyl ring dynamics already far belowT _g.     

Structure–property relationships in polyamides containing cyclohexylene or phenylene structures

Polyamides were prepared from linear, aliphatic dicarboxylic acids of six to twelve carbon atoms with 1,4-cyclohexanebis(methylamine), 1,4-cyclohexanebis (ethylamine), p-xylylenediamine, and p-phenylenebis(ethylamine). Melting points, glass transition temperatures, densities, and moisture regains were compared for the polymers to determine the relative effect of the cyclohexylene and phenylene linkages. While polyamides containing the trans-cyclohexylene group possessed higher glass transition temperatures than their aromatic counterparts, melting behavior was not as consistent. The odd-even rule, which states that polyamides with an even number of methylene linkages between the ring and the functional group melt higher than those with an odd number of such linkages, was violated in the cycloaliphatic systems. The T_g of ring-containing polyamide fibers was not dependent solely upon ring concentration, but was influenced by the molecular fit of the ringed intermediate in the polymer chains. Molecular fit appears to affect the T_g and melting point of alicyclic polyamides to a greater extent than the aromatic analogs. Differences in T_g, both within and among the polymer series, was not explained by either density or the degree of crystallinity.     

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (T_g) on film thickness. Sizeable reduction of T_g was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of T_g on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the T_g or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Effect of dopant structure on refractive index and glass transition temperature of polymeric fiber‐optic materials

Graded‐index plastic optical fibers, composed of doped polymers, have advantages over conventional glass optical fibers, but need to be developed further for practical application. Here, a variety of aromatic sulfide dopants were synthesized, and their effects on the refractive indexes and glass transition temperatures (T_g) of poly(methyl methacrylate) and methyl 2‐chloroacrylate/2,2,2‐trichloroethyl methacrylate copolymers were studied. While polymers containing large dopants exhibited relatively high refractive indices, their T_g values were low, making these materials unsuitable for graded‐index plastic optical fiber applications. Six dopants yielded polymers that exhibited higher T_g values than the conventionally used (diphenyl sulfide)‐doped polymer. The dopant dibenzothiophene, in particular, yielded polymers with the highest refractive indexes and T_g values, and polymers containing (phenylthio)benzene dopants also performed well. Copyright © 2013 John Wiley & Sons, Ltd.     

Evidence of dynamic heterogeneity as obtained from dielectric and brillouin light-scattering studies of poly(n-hexylmethacrylate)

We report dielectric relaxation and Rayleigh-Brillouin spectroscopic measurements on the side chain polymer poly(n-hexylmethacrylate), PHMA (T_g = 268 K), exhibiting a broad glass transition region. The dielectric loss curves can be represented by single Havriliak-Negami functions in the temperature range of 260–450 K. The width of the distribution relaxation function is a decreasing function of temperature up to T = 333 K ≊ 1.24 × T_g and remains virtually constant above that temperature. This is interpreted as marking the merging of the α-process with a slow β-relaxation in agreement with the value of the cooperativity length associated with the α-mode. Hence above that temperature, the relaxation times confirm well to an Arrhenius temperature dependence. The hypersonic dispersion deduced from the Brillouin spectra (210–550 K) surprisingly peaks at temperatures near T_g which bears no relation to the main α-relaxation. This structural relaxation is rather associated with the side hexyl group motion showing striking resemblance with the hypersonic dispersion in molecular liquids. It is conceivable that the observed damping in PHMA is dynamically related to the internal plasticization effect of the hexyl group. © 1996 John Wiley & Sons, Inc.     

A study of enthalpic relaxation of a liquid crystal

A liquid crystal, BL038, which was observed not to crystallize, has a glass transition at 215 K and a nematic to isotropic transition at 380 K. Samples aged below the glass transition at various temperatures T _a, exhibited an endotherm at the transition which developed with extent of ageing time, t _a. We attribute this endotherm to the relaxation of the glass towards the equilibrium liquid. The progress of the relaxation process was measured using differential scanning calorimetry. On subsequent reheating, the aged glass showed an apparent shift in the glass transition to higher temperatures. The endotherm was used to define the extent of enthalpic relaxation and the maximum value observed was found to increase initially then decrease, with the extent of undercooling from the glass transition temperature, Δ T, passing through a maximum for a Δ T = 15 K. From the temperature dependence of the relaxation times, an apparent activation enthalpy for the relaxation process of 85 ± 10 kJ mol^-1 was determined. The small value of the activation enthalpy compared with that found in the ageing of polymers reflects differences in the molecular species involved in relaxation processes.     

The state of high global chain orientation but nearly random segmental orientation of amorphous polymers

In the rubbery state of amorphous polymers under uni-axial drawing the global chain orientation will relax in orders of magnitude slower than the relaxation of local segmental orientation. When this state of hot drawn sample is frozen at temperatures lower than its glass transition, T_g, an amorphous state with high global chain orientation but nearly random segmental orientation (GOLR) could result. Experimentally the GOLR state of amorphous polymers is easily realized by uni-axial drawing the polymer at temperatures 20–30°C above its T_g. The characteristic features of a GOLR state are i) large recovery of the elastic deformation of global chain on being heated to temperatures above T_g, ii) very small birefringence and IR dichroism, iii) nearly isotropic sonic and ultrasonic velocity of propagation, iv) nearly isotropic WAXD pattern, while it shows v) pronounced anisotropy in stress-strain behavior for large deformations and vi) appreciable anisotropy of thermal conductivity and of microwave dielectric properties. Concrete examples with detailed experimental results will be reviewed.     

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