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.     

Influence of temperature on the ultrasonic degradation of poly(vinyl acetate) and poly(vinyl chloride)

This study investigates the effect of temperature on the ultrasonic degradation of polyvinyl acetate and polyvinyl chloride in chlorobenzene. The time evolution of molecular weight distribution was determined using gel permeation chromatography. Continuous distribution kinetics was used to obtain the degradation rate coefficients. The rate coefficients decrease with increasing temperature, and this is attributed to the increase of vapor pressure and the decrease of kinematic viscosity. The degradation rate coefficient also changes sharply near the glass transition temperature, indicating that this factor may play a role in the degradation process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 12: 2818–2822, 2003     

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.     

Study of poly(vinyl chloride) interfaces using slow positron beams

Elemental and X‐ray fluorescence analyses were performed to determine the chemical compositions of insulator and jacket poly(vinyl chloride) (PVC) samples. In addition, differential scanning calorimetry (DSC) measurement was performed to determine their glass‐transition temperatures (T_g) and melting points. The effect of additives on the two investigated samples, as well as on a pure PVC sample, was studied using Doppler‐broadening energy spectra coupled with the slow positron beam technique. Significant variation in the S parameter as a function of positron implantation energy and depth from the surface to the bulk was observed in all samples. The S parameter increased at a very low positron energy (< 1 keV), saturated to about 6 keV, and then decreased up to 27 keV. The S parameter (ΔS) changed to the extent of the change in the breadth of the distribution of free‐volume defects, which was larger in the jacket PVC sample, which had more additives, than in the insulator PVC sample. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3789–3793, 2006     

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.     

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.     

Combined effect of relative humidity and temperature on dynamic viscoelastic properties and glass transition of poly(vinyl alcohol)

On the basis of the experimental studies on viscoelastic properties of poly vinyl alcohol (PVA) films at various relative humidity (RH) and temperature conditions by dynamic mechanical analysis (DMA), the influence of both temperature and RH on the glass transition are discussed and an improved property model is developed to relate the dynamic modulus to RH and temperature. The results indicate that (1) with increasing the RH, the storage modulus of PVA decrease remarkably, while both loss modulus and tanδ sharply increase to reach the peak and then markedly drops. The intensity of this variation is highly dependent upon temperature. (2) Moisture increase will cause the glass transition of PVA at isothermal condition and the transition point can be detected by glass transition relative humidity (RH_g) that obtained by isothermal RH scans. (3) Similar to the relationship between T_g and RH, the RH_g of PVA vary linearly with temperature. The state diagram of RH_g versus temperatures is nearly consistent with that of T_g versus RH. (4)The present equation based on model of Mahieux and Reifsnider (Mahieux and Reifsnider, Polymer 2001, 42, 3281) can predict well the dynamic modulus of PVA at various RHs and temperatures. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3161–3167, 2013     

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).     

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.     

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.     

Alpha relaxation study in poly(vinyl chloride)/poly(ethyl methacrylate) blends using thermally stimulated currents

Thermoelectrets of poly(vinyl chloride) (PVC) and poly(ethyl methacrylate) (PEMA) blends were prepared using the conventional thermal poling method. α‐Relaxation in this polyblend was investigated using thermally stimulated depolarization currents (TSDC). The global spectra of the polyblends revealed that the two polymers are not completely compatible. An Eyring relationship was verified through the linear relation between the activation enthalpy ΔH and the activation entropy ΔS. In addition, the thermal sampling (TS) data were used to determine the compensation parameters such as compensation temperature T_c and compensation time (τ_c). These parameters were used to calculate the density of disorder (DOD) for all samples, which was found to be close to 32%. © 2000 Society of Chemical Industry     

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.     

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.