Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature T_g of molecular (non-polymeric) glass formers correlates with molecular mass M as T_g(M) ∝ M^α, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T_g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T_g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T_g(M) dependence in polymers begin to deviate from the universal T_g(M) correlation of molecular glasses and eventually saturates at some polymer specific T_g∞ value. We conclude that at least a substantial part of T_g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T_g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T_g(M) dependence common with molecular glasses.     

Quantitative evaluation of the Gibbs theory of the glass transition

The utility of a reduced parametric plot of glass transition (T_g/T_g∞) versus the reciprocal degree of polymerization ($\text{10}{}^3\text{/}\text{P}\text{?}$) is reviewed and illustrated for the case of the Gibbs theory of the glass transition. These results show that the number of lattice sites occupied per mer (n) affects T_g more than variations in the fractional free volume at T_g(V_o). Traditionally a variation of the Gibbs theory (n = 1) has been used to explain the $\text{P}\text{?}$ dependence of T_g for poly(methyl methacrylate) (PMMA). However, statistical analyses of all available PMMA data sets show that there is an interrelation between n and V₀ at the 0.05 level such that many acceptable solutions occur, one of which is at V₀ = 0.025 and n = 1.25. Therefore, a unique solution may not be obtained unless a free volume is either (1) assumed as Gibbs and others had routinely done, or, more properly, (2) calculated from the PVT equation of state. In the latter case, however, knowledge of the ratio of hole energy to flex energy (r) is required. With only subjective evaluations previously available, the statistical methodology is presented as an alternative, objective approach to compare T_g data with theory.     

Glass transition and physical ageing in plasticized poly(vinyl chloride)

Several samples of poly(vinyl chloride) both unplasticized and plasticized with dioctyl phthalate, have been examined by differential scanning calorimetry. It was observed that, while the glass transition temperature T_g decreased as expected with increasing plasticizer content, a small portion of the sample appeared to be resistant to the plasticizer. This was manifest in the appearance of a second T_g corresponding to the unplasticized sample which remained unaffected by addition of plasticizer. The ageing behaviour of the samples was also examined using enthalpy relaxation measurements and it was observed that the presence of plasticizer accelerates the ageing process, probably due to the fact that there is greater mobility of the chains in the plasticized samples.     

Thermal analysis of the glass transition of plasticized poly(vinyl chloride)

Earlier studies by differential scanning calorimetry showed anomalous behavior of T_g with varying amounts of dioctyl phthalate, while by dynamic mechanical analysis 2 T_g's were found over a limited concentration range. In this study, two adjacent T_g's were found over the complete range of concentrations of dioctyl phthalate or dioctyl adipate, changing in relative contributions over the range. The results are shown to be consistent with the earlier work and to fit the Couchman model more closely than other models tested.     

Miscibility and interactions in poly(n-propyl methacrylate)/poly(vinyl alcohol) blends

Poly(n-propyl methacrylate) (PPMA) is miscible with poly(vinyl alcohol) (PVA) over the whole composition range as shown by the existence of a single glass transition temperature in each blend. The interaction between PPMA and PVA was examined by Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance spectroscopy. The interactions mainly involve the hydroxyl groups of PVA and the carbonyl groups of PPMA. The measurements of proton spin-lattice relaxation time reveal that PPMA and PVA do not mix intimately on a scale of 1-3 nm, but are miscible on a scale of 20-30 nm. A small negative interaction parameter value has been obtained by melting point depression measurement.     

Glass transition and structural relaxation in semi-crystalline poly(ethylene terephthalate): a DSC study

The aim of this work is to determine the relaxation times of the cooperative conformational rearrangements of the amorphous phase in semi-crystalline poly(ethylene terephthalate) (PET) and compare them with those calculated in amorphous PET. Samples of nearly amorphous polymer were prepared by quenching and samples with different crystallinity fractions were prepared from the amorphous one using cold crystallisation to different temperatures. The differential scanning calorimetry (DSC) thermograms measured on samples rapidly cooled from temperatures immediately above the glass transition show a single glass transition which is much broader in the case of high-crystallinity samples than in the amorphous or low-crystallinity PET. To clarify this behaviour, the samples were subjected to annealing at different temperatures and for different periods prior to the DSC measuring heating scan. The thermograms measured in samples with low crystallinity clearly show the existence of two amorphous phases with different conformational mobility, these are called Phases I and II. Phase I contains polymer chains with a mobility similar to that in the purely amorphous polymer, while Phase II shows a much more restricted mobility, probably corresponding to conformational changes within the intraspherulitic regions. The model simulation allows to determine the temperature dependence of Phase II relaxation times, which are independent from the crystallinity fraction in the sample and around two decades longer than those of the amorphous polymer at the same temperature.     

Glass transition and structural relaxation of nano-particle aggregates of atactic poly(methyl methacrylate) formed in microemulsions

Atactic poly(methyl methacrylate) (a-PMMA) nano-particles were prepared in three-component microemulsions using the cationic surfactant cetyltrimethylammonium bromide. To analyze the thermal history, the sub-T_g annealed sample was subsequently measured at a faster heating rate. The a-PMMA prepared in microemulsions shows higher glass transition and higher structural relaxation than those of bulk sample of the same tacticity and the same molecular weight. When the annealing time is longer, additional enthalpy relaxation and glass transition were observed at higher temperatures due to partial self-aggregation of a-PMMA chains, which was proved by spectroscopic studies.     

Preparation,glass transition temperature,and δ relaxation effect of poly(vinyl isonicotinate)

Poly(vinyl isonicotinate) was prepared by esterification of poly(vinyl alcohol) with isonicotinoyl chloride in pyridine solution. Poly(vinyl nicotinate) was also prepared. Density, refractive index, and glass transition of the polymers were determined. The low temperature dynamic mechanical properties of poly(vinyl isonicotinate) are characterized by a relaxation effect which is associated with thermally excited motions of the pyridine rings (δ relaxation). The strong displacement of such a phenomenon toward higher temperatures, with respect to poly(vinyl benzoate), is interpreted as due to the elevated polarity of the pyridine ring.     

Poly(vinyl chloride), Part II: Effect of polymerization temperature and molecular weight on the glass transition and melting point of poly(vinyl chloride)

A detailed study was made of the thermal behavior of poly(vinyl chloride) (PVC) homopolymers, whose chain length and structure were independently manipulated. Normal PVC polymerizations are made at temperatures, the selection of which determines the polymer chain length. In the case of PVC, this choice of temperature also determines its syndiotacticity, and so, its glass transition temperature, and in many ways, its processibility. The ability to understand how to control chain structure and length in PVC is critical in designing polymers for selected specific end use conditions. The thermal spectrum is known to be one measure of how polymers will perform. This study reveals how such controlled chain length/structure PVCs are characterized by thermal methods, and how one can separate individually the effects of syndiotacticity and molecular weight on thermal properties of poly(vinyl chloride).     

Dynamic relaxation characteristics of polymer nanocomposites based on poly(ether imide) and poly(methyl methacrylate)

The dynamic relaxation characteristics of poly(ether imide) and poly(methyl methacrylate) nanocomposites based on native and surface-modified (i.e., hydrophobic) fumed silicas were investigated by dynamic mechanical analysis and dielectric spectroscopy. The nanocomposites displayed a dual glass transition behavior in the dynamic mechanical studies encompassing a bulk polymer glass transition (close to T_g for the unfilled polymer), and a second, higher-temperature transition reflecting relaxation of polymer chain segments constrained owing to their proximity to the particle surface. The position and intensity of the higher-temperature transition varied with particle loading and surface chemistry, and reflected the relative populations of segments constrained or immobilized at the particle-polymer interface. Dielectric measurements, which were used to probe the time-temperature response across the local sub-glass relaxations, indicated no variation in relaxation characteristics with particle loading.     

Quantitative study of the annealing of poly(vinyl chloride) near the glass transition

Poly(vinyl chloride) displays a normal DSC of DTA curve for the glass transition when quenched from above its T_g. However if cooled slowly or annealed near the glass transition temperature, a peak appears on the DSC or DTA curve at the T_g. In this paper quantitative studies of the time and temperature effects on the production of this endothermal peak during the annealing of PVC homopolymer and an acetate copolymer are presented. The phenomenon conforms to the Williams, Landell, and Ferry equation for the relaxation of polymer chains, the rate of the peak formation becoming negligible at more than 50°C below T_g. The energy difference between the quenched and annealed forms is small. For a PVC homopolymer annealed 2 hr at 68°C, which is T_g ?10°C, the difference is 0.25 cal/g. For a 13% acetate copolymer of PVC similarly annealed, the difference is 0.36 cal/g. The measured rates of the process give a calculated activation energy of 13–14 kcal/mole for PVC homopolymer and copolymer. This appearance of a peak on the T_g curve for a polymer when annealed near the glass temperature appears to be a general phenomenon.     

The effect of confinement in nanoporous polymers on the glass transition temperature

The glass transition temperature (T_g) of nanoporous polyetherimide (PEI) and PEI thin films was investigated. The T_g decreased from its bulk value in both of these confined systems. Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances in the nanoporous PEI. A quantitative analogy between the nanoporous PEI and PEI thin films is proposed through an equivalence of nearest neighbor pore-to-pore distances and thin film thickness. The effect of confinement is believed to be due to the interface regions, which possess higher chain mobility than the bulk. When these high mobility interface regions are sufficiently close together, the excess mobility at the interface region affects the dynamics of the system by restraining percolation of the slow domains resulting in the observed decrease in T_g.     

The effect of the glass transition temperature on the toughness of photopolymerizable (meth)acrylate networks under physiological conditions

The purpose of this study is to evaluate how the toughness of photopolymerizable (meth)acrylate networks is influenced by physiological conditions. By utilizing two ternary (meth)acrylate networks, MA-co-MMA-co-PEGDMA and 2HEMA-co-BMA-co-PEGDMA, relationships between glass transition temperature (T_g), water content and state, and toughness were studied by varying the weight ratio of the linear monomers (MA to MMA or 2HEMA to BMA). Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal behavior and water content as a function of either MA or 2HEMA concentration while tensile strain-to-failure tests were performed at 37 °C to determine network toughness. Both networks exhibited a maximum in toughness in PBS in the composition corresponding to a T_g close to the testing temperature. This toughness maximum was achieved by adjusting the glass transition temperature and/or hydrophilicity through changes in chemistry. These relationships may be utilized to design tough photopolymerizable networks for use in mechanically rigorous biomedical applications.     

Effect of confinement on the relaxation behavior of poly(ethylene oxide)

It is widely thought that confinement of an amorphous polymer alters the chain mobility, which affects the temperature and intensity of the glass transition. The present study sought to determine whether the same effects extend to semicrystalline polymers. Confinement was achieved by forced assembly of hundreds of alternating layers of poly(ethylene oxide) (PEO) with either poly(ethylene-co-acrylic acid) or polystyrene. The confinement gradually reduced the intensity of the PEO β-relaxation as the layer thickness decreased from the microscale to the nanoscale. By considering the changes in crystalline morphology that accompanied layer confinement, it was possible to completely account for the reduction in relaxation intensity using standard mechanical models. The viscoelastic behavior of the amorphous phase was satisfactorily represented by a modified standard linear solid (SLS). The amorphous and crystalline contributions were combined using a combination of parallel and series coupling in accordance with the Takayanagi model. No adjustment in the viscoelastic parameters of the modified SLS was required, indicating that there was no significant change in amorphous chain dynamics even in layers as thin as 45 nm.     

Something about amber: Fictive temperature and glass transition temperature of extremely old glasses from copal to Triassic amber

In previous work we successfully used a stable Dominican amber to investigate the sub-glass transition dynamics of an organic glass with extremely low fictive temperature. The results provided an incentive to seek other stable ambers with lower values of fictive temperature. In the present work, a series of fossil resins from different locations, and ages ranging from approximately 100 years–230 million years, has been investigated using differential scanning calorimetry. The measurement results show the thermal signatures for each fossil sample: copals are unstable, and have lower glass transition temperatures than the amber samples. For the amber samples, there is not a systematic age dependence for the amber's glass transition temperature. Furthermore, even for the same sort of amber, the thermal properties can be different for different samples. The stability of the fossil resins was studied and glass transition temperatures were determined. For the stable samples, fictive temperature was also determined.     

Dynamic infrared measurements of poly(vinyl chloride) in the vicinity of the glass transition

The temperature dependences of some selected infrared bands of poly(vinylchloride) in the vicinity of the glass transition were examined. The intensities of 1333 and 1427 cm^?1 bands changed discontinuouly at T_g; on the other hand, 2920 cm^?1 did not show any significant changes at this temperature. From these results, the nature of the temperature dependence of infrared intensity of the polymer was discussed. The procedure introduced in this work offers a rapid method for the determination of the T_g as compared with the conventional infrared methods.     

Thickness and composition dependence of the glass transition temperature in thin random copolymer films

Glass transition temperatures (T_g) of thin poly(styrene-co-methyl methacrylate) and poly(2-vinyl pyridine-co-styrene) films coated on a native oxide surface of Si wafer (100) were measured by ellipsometry. The thickness dependence of T_g can be properly fitted by previously suggested equation developed for homopolymers, based upon a continuous multi-layer model, although one component in thin random copolymer films demonstrates a slightly favorable interaction between a substrate and thin film, and another demonstrates a strongly favorable interaction. Surface and interface have a strong influence on T_g of thin film coated on substrate: the surface has the effect of reducing T_g, whereas the interface increases the T_g according to the degree of interaction between a substrate and thin film. This degree of interaction can be quantified as an interaction parameter (k), and is dependent on the composition of random copolymers. For the estimation of k values of thin random copolymer films, we proposed a parallel type additive function (1/k_ran=w₁/k₁+w₂/k₂) where w is a weight fraction of component.     

Correlation of the glass transition temperature of plasticized PVC using a lattice fluid model

A model has been developed to estimate the glass transition temperature of polymer+plasticizer mixtures (up to 30 wt% plasticizer). The model is based on the Sanchez-Lacombe equation of state and the Gibbs-DiMarzio criterion, which states that the entropy of a mixture is zero at the glass transition. The polymers studied included polystyrene and poly(vinyl chloride). The plasticizers studied included a wide range of chemicals from methyl acetate to di-undecyl phthalate. The model qualitatively accounted for the effect of different plasticizers on the mixture glass transition temperature.     

The glass transition temperature of poly(N-vinyl pyrrolidone) by differential scanning calorimetry

Previously a wide range of values have been reported for the glass transition temperature, T_g, of poly(N-vinyl pyrrolidone), PVP, and it was suggested that lower values are due to variable uptakes of water caused by the hygroscopic nature of the polymer. Now it has been found that there are large variations in T_g, even in carefully dried specimens of PVP. Other factors found to influence T_g are residual monomer and the molecular weight of PVP. Polymers prepared by bulk polymerization, either by γ-irradiation or by heating with 2-azobisisobutyronitrile, have much lower values of T_g than dried ones prepared containing 30% water. The difference is mainly due to depression of T_g by residual monomer which, in the absence of water during polymerization, fails to react completely because of conversion to a glassy state. An unexplained observation is that even when all residual monomer has been removed, polymers prepared by bulk polymerization still have a lower T_g than would be expected from their molecular weight.