Elevation of the glass transition temperature in flexible‐chain semicrystalline polymers

In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (T_g) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine T_g in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. T_g is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007     

New simple method of measuring the surface glass transition temperature of polymers

A lap‐shear joint mechanical testing method has been probed to measure the surface glass transition temperature (T) of the thick bulk films of high‐molecular‐weight polymers. As T, the temperature transition “occurrence of autoadhesion–nonoccurrence of autoadhesion” has been proposed. The influence of chain flexibility, of molecular architecture, of polymer morphology, and of chain ends concentration on the T has been investigated. The correlation between the reduction in T with respect to the glass transition temperature of the bulk (T) and the intensity of the intermolecular interaction in the polymer bulk in amorphous polymers has been found. The effect of surface roughness on T has been discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2012–2021, 2010     

Free volume microstructure of amorphous polymers at glass transition temperatures from positron annihilation spectroscopy data

The analysis of annihilation characteristics of ortho-positronium at conventional calorimetric glass transition temperatures for a series of amorphous polymers reveals empirical correlations of average lifetime of o-Ps , and of its product with a relative intensityI _3g with appropriateT _g ^DSC values. These trends in terms of free volume mean that both the average size of free volume hole entityv _hg and the fractional free volume grow with increasingT _g ^DSC . The results are discussed considering the chemical microstructure as well as possible mechanisms acting in glass transition. A relation is indicated between geometric and flexibility characteristics of chains and thev _hg andf _g parameters of free volume microstructure on the one side and potential motional processes responsible for solidification of the amorphous system on the other side.     

Effect of benzenesulfonamide plasticizers on the glass‐transition temperature of semicrystalline polydodecamide

The effect of various benzenesulfonamide (BSA) plasticizers on the amorphous phase of semicrystalline polydodecamide (PA‐12) has been investigated. MonoBSAs appear as efficient glass‐transition temperature (T_g) depressors because of their miscibility with the host polyamide (PA), low glass transition, and small molecule size. PA‐12's T_g shifts from 50 to about 0 °C at 20 mol % of the most efficient molecules. Comparatively, the more bulky bisBSAs appear to induce less important absolute T_g decreases (30 K at 20 mol %), although these appear as more important when considering the polymer T_g to plasticizer T_g difference. This unexpected observation could be ascribed to both the amide‐sulfonamide interactions and the sterically generated disorder within the polyamide because of the plasticizer molecule's size. Phase‐separation behavior of BSA plasticizers within the host PA has also been investigated. Crystalline phenyl‐SO₂NH₂, for instance, dephased beyond 20 mol % in PA‐12, forming distinct 1–2 micrometer wide crystalline domains as a result of its high propensity to crystallize upon cooling from the melt. By contrast, slow crystallizing N,N‐dimethylBSA, which lacks any specific interaction for PA‐12, remained nevertheless dispersed at a molecular level (metastable state, no phase separation) when vitrification of the host PA‐12 amorphous phase occurred on cooling. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2208–2218, 2002     

Performance of gelled-type dye-sensitized solar cells associated with glass transition temperature of the gelatinizing polymers

Poly(methyl acrylate) (PMA), poly(vinyl acetate) (PVAc) and poly(n-isopropylacrylamide) (PNIPAAm) with their respective T_g of 6, 32, and 145 °C were employed to gel the LiI/I₂/tertiary butylpyridine electrolyte system for preparation of the gelled-type dye-sensitized solar cells (DSSC). The light-to-electricity conversion efficiencies of DSSCs gelled by PMA, PVAc, and PNIPAAm were 7.17%, 5.62%, and 3.17%, respectively under simulated AM 1.5 sunlight irradiation, implying that utilizing the polymer of lower T_g to gel the electrolytes leaded to better performance of the DSSCs. Their short-circuit current density and IPCE also showed the similar trend. Electrochemical impedance spectroscopy of the gelled DSSCs revealed that utilizing the polymer of lower T_g resulted in lower impedance associated with the Nernstian diffusion within the electrolytes. The results were consistent with the observation that the molar conductivity of gelled electrolytes was higher as the polymer of lower T_g was applied, which can be justified by Vogel-Tammann-Fulcher (VTF) equation.     

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.     

Thermal stability and high glass transition temperature of 4-chloromethyl styrene polymers bearing carbazolyl moieties

A new styrene derivative monomer, 4-(N-carbazolyl)methyl styrene (CzMS), was synthesized by reacting 4-chloromethyl styrene with carbazole in the presence of sodium hydride. Then, CzMS was homopolymerized and copolymerized with different monomers such as methyl methacrylate (MMA), ethyl methacrylate (EMA), methyl acrylate (MA), ethyl acrylate (EA) and n-butyl acrylate (BA) by free radical polymerization method in N,N-di-methylformamide (DMF) solution at 70 ± 1 °C using azobisisobutyronitrile initiator to give the copolymers I-V in good yields. The structure of all the resulted polymers was characterized and confirmed by FT-IR, ¹H NMR and ¹³C NMR spectroscopic techniques. The average molecular weight and glass transition temperature of polymers were determined using gel permeation chromatograph (GPC) and differential scanning calorimeter (DSC) instruments, respectively. It was found that these polymers with carbazole moieties have high thermal stability and the presence of bulk carbazole groups in polymer side chains leads to an increase in the rigidity and glass transition temperature of polymers.     

Application of free‐volume theory to self diffusion of solvents in polymers below the glass transition temperature: A review

Theories based on free‐volume concepts have been developed to characterize the self and mutual‐diffusion coefficients of low molecular weight penetrants in rubbery and glassy polymer‐solvent systems. These theories are applicable over wide ranges of temperature and concentration. The capability of free‐volume theory to describe solvent diffusion in glassy polymers is reviewed in this article. Two alternative free‐volume based approaches used to evaluate solvent self‐diffusion coefficients in glassy polymer‐solvent systems are compared in terms of their differences and applicability. The models can correlate/predict temperature and concentration dependencies of the solvent diffusion coefficient. With the appropriate accompanying thermodynamic factors they can be used to model concentration profiles in mutual diffusion processes that are Fickian such as drying of coatings. The free‐volume methodology has been found to be consistent with molecular dynamics simulations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Temperature modulated calorimetry and dielectric spectroscopy in the glass transition region of polymers

The results from temperature modulated DSC in the glass transition region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent glass transition temperature can be fitted with a VFT-equation. The transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The glass transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

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.     

Determination of the glass transition temperature in thin polymeric films used for microelectronic packaging by temperature-dependent spectroscopic ellipsometry

We characterized the glass transition temperature T_g of thin polyimide films by temperature-dependent spectroscopic ellipsometry and compared the results to DSC measurements of the bulk polymer. The effect of the curing temperature on T_g and the thermal expansion α(T) was analyzed. An improved ellipsometric data evaluation was used to get most precise and reliable T_g data. T_g increased with increasing curing temperature, while the bulk T_g was considerably lower than the thin film T_g. Both observations are attributed to the temperature sensitive release of the imidization by-product 2-hydroxyethyl methacrylate (HEMA) and crosslinker components as well as decomposition products from the material. Variation in the curing temperatures of 230–380 °C led to an increase in the T_g of 34 °C.     

Polystyrene with different topologies: Study of the glass transition temperature in confined geometry of thin films

The glass transition behaviour of polystyrene (PS) with systematically varied topologies (linear, star-like and hyperbranched) confined in nanoscalic films was studied by means of spectroscopic vis-ellipsometry. All applied PS samples showed no or only a marginal depression in glass transition temperature T_g in the order hyperbranched PS (5 K) > star-like PS (3 K) > linear PS (0 K) for the thinnest films analyzed. The T_g behaviour was accompanied by the observation of the film density in dependence of film thickness. A maximum decreased density of about 7% for hyperbranched PS and 5% for star-like PS and again no deviation in density of bulk was found for linear PS. Accordingly, we deduce from these results considering an experimental accuracy of about ± 2 K for T_g and up to ±3% for film density, that the polymer topology only barely influences T_g in the confinement of thin films.     

Interactions in a blend of two polymers greatly differing in glass transition temperature

Blends of poly(N‐methyldodecano‐12‐lactam) PMDL with poly(4‐vinyphenol) PVPh have been studied by the DSC and ATR FTIR methods. The difference in glass transition temperature T_g between the components is 206 °C. A single composition‐dependent T_g suggests miscibility of the system, that is, homogeneity on the scale of about 10 nm. Fitting of the equation of Brostow et al. to the T_g data indicates relatively strong specific interactions and high complexity of the system. The Schneider's equation applied separately to low‐ and high‐PVPh regions provides good agreement with experiment; the calculated curves cross at the point of PVPh weight fraction 0.27. In the low‐PVPh region, the analysis indicates weak interactions with predominance of segment homocontacts and strong involvement of conformational entropy. In the high‐PVPh region, strong specific interactions predominate and entropic effects are suppressed. Composition dependences of the heat capacity difference at T_g and the width of glass transition indicate strong interactions in the system and existence of certain heterogeneities on segmental level, respectively. According to ATR FTIR, hydrogen bonds between PVPh as proton donor and PMDL as proton acceptor induce miscibility in blends of higher PVPh content (above about 0.28 weight fraction). In low‐PVPh blends, it is conformational entropy that enables intimate intermolecular mixing. Hydrogen bonds adopt several (distorted) geometries and are on average stronger than average hydrogen bonds formed in self‐associating PVPh. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

An analysis of the thermal aging behaviour in high-performance energetic composites through the glass transition temperature

Differential scanning calorimetry (DSC) was used to analyze the thermal aging behaviour in energetic composite materials where a hydroxyl-terminated polybutadiene (HTPB)/isophorone diisocyanate elastomer is the polymeric matrix. Different parameters from the analysis of the glass transition, such as the glass transition temperature (T_g), were used in order to monitor this isothermal aging at 65 °C during a total time of 3000 h, finding an increasing and broadening T_g. In addition, the accelerated aging behaviour of these materials was also studied by a classical method, based on the change of mechanical properties such as those of Young's modulus or strain at break. The correlation between both methodologies was examined, demonstrating that an analytical technique such as DSC allows the evaluation of the actual state of composite solid propellants with a small sample and a straightforward measurement.     

Moisture effects on the glass transition and the low temperature relaxations in semiaromatic polyamides

The influence of moisture absorption on the primary (glass) transition (T_a or T_g) and the low temperature relaxations of semiaromatic amorphous polyamides synthesized by isomeric aliphatic diamine and metha or para oriented phthalicdiacids has been investigated by means of differential scanning calorimeter (DSC) and dynamic mechanical thermal analyser (DMTA). The glass transition of semiaromatic polyamides was lowered due to the water absorption, and the β and the γ relaxations were as well. From the observed T_g and the difference in the heat capacity, the calculated T_g depression per 1 wt % water content was 12.3 K and the result was in good agreement with the experimental data. The depression of the glass transition may be expressed by the same manner as the plasticization of nylon 6 by water. The depressed β relaxation observed in the specimen containing a few percent of moisture was splitted into two transitions due to the reduction of water content, of which one was the elevation of the T_β and another was the simultaneous appearance of the T_γ, and then the single T_γ solely was observed for the completely dried specimen. The T_γ seemed to be merged into or not to be observed by the large and broad T_β transition when the sample was governed by a few percent of water, then it was emerged from the T_β due to water desorption. Thus, the T_β is believed to arise from the intermolecular hydrogen bonding between water molecules or between water and amide groups in wet polyamides. In addition, the γ relaxation originated from the peptide groups is attributable to the inter- and intramolecular hydrogen bonding between amide groups. © 1997 John Wiley & Sons, Inc. J Polyn Sci B: Polym Phys 35: 807–815, 1997     

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     

Determination of the glass transition temperature of polystyrene,poly(vinyl chloride) and poly(methyl methacrylate) using gas chromatography

The glass transition temperatures, Tg, of polystyrene, poly (vinyl chloride) and poly(methyI methacrylate) have been determined from gas chromatographic measurements using n-hexane, n-heptane, meta-xylene and para-xylene solvents. The glass transition temperatures were detected on the z-shaped retention diagrams which were produced from the plot of the logarithm of the specific retention volumes of the above-mentioned solvents against the reciprocal of temperature, i.e. log V${}_{\text{g}}^{\text{º}}$ vs. 1/T. The glass transition temperature is specified by the temperature where the slope of log V${}_{\text{g}}^{\text{º}}$ vs. 1/T changes abruptly. The observed glass transition temperature of polystyrene produced by this technique was found to be in good agreement with those produced by other techniques such as the differential scanning colorimeter. The industrial importance of the glass transition temperature, T_g, might be due to the dramatic changes in the physical properties of the polymer, such as hardness and elasticity, which take place in the vicinity of this temperature. However, perfectly crystalline polymers do not exhibit glass transitions, because their chains are incorporated in regions of three-dimensional order, called crystallites. Completely amorphous polymers and semi-crystalline polymers usually exhibit both glass transition and melting.     

Unveiling the impact of regioisomerism defects in the glass transition temperature of PVDF by the mean of the activation energy

Alternating two groups, CH₂ and CF₂, of very different polarities along the backbone chain of polyvinylidene fluoride (PVDF) leads to very interesting properties, such as ferroelectricity. However, these properties are affected by the presence of regioisomerism defects (monomer inversion) that appear during the synthesis. In this study, the influence of these defects on the glass transition temperature (T _g) is regarded. Using molecular simulation, we investigated their plasticizer effect. To highlight their role in modifying polymer chain relaxation, the activation energy (E _a) related with conformational transitions is computed. We show that a clear linear relationship can be established between E _a and T _g. E _a is then separated into different contributions associated with each type of bond. Mobility of individual segments can thus be accounted separately from the mobility of the whole chain. As a consequence, T _g of the alternate copolymer ethylene‐tetrafluoroethylene (E‐TFE), an isomeric polymer of PVDF, can be deduced. T _g of this copolymer is still source of discussion in the literature. The resulting value, ?97 °C, is found in good agreement with some experimental data, and also with T _g extracted from simulated dilatometry. Consequently, local mobility of a polymer chain can be retrieved from the whole mobility, and vice versa. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 419–426     

On the glass transition temperature of styrene-n-butyl methacrylate copolymers in dependence on chemical composition,molecular weight,and mixtures

For statistic copolymers of styrene and n-butyl methacrylate, the relation between the glass transition temperature and the chemical composition or molecular weight of the copolymers has been determined. Further, the dependence of the glass transition temperature on the composition of binary and ternary blends from statistical poly (styrene-co-n-butyl methacrylates) of a nearly equal chemical composition but a very different molecular weight has been studied. Among several equations considered for the correlation between glass transition temperature and composition of the mentioned copolymers with relatively low molecular weights, the Gordon/Taylor and Couchman equations gave the best agreement with the experimental results. For the glass transition temperature of poly(styrene-co-n-butyl methacrylate) with an n-butyl methacrylate content of about 30 wt % in dependence on the molecular weight, the Kanig-Ueberreiter and Fox-Flory equations proved to be useful for the examined molecular weight range. The glass transition temperatures of the polymer blends have been studied for a low/high-molecular component system, a system of two low-molecular components, as well as for systems with a third component. The glass transition temperatures of the mixtures frequently exceeded those of their individual components. © 1994 John Wiley & Sons, Inc.