The chemical dehydrochlorination of poly(vinyl chloride), 1. Studies on tetrahydrofuran solutions

The chemical dehydrochlorination of poly(vinyl chloride) was studied in solution in tetrahydrofuran. Two materials, a mass polymer of 55% syndiotacticity and a low temperature polymer of 63% syndiotacticity, were dehydrochlorinated with potassium hydroxide and with potassium tert-butoxide. The dehydrochlorination products were characterised by their chlorine contents and by UV/visible, IR and Raman spectroscopy, as a function of reaction time. The course of the reaction was found to be substantially independent of tacticity and the dehydrochlorinating reagent. Initially, the reaction follows a similar path to that occurring during photo- and thermal degradation, but as the dehydrochlorination continues, longer conjugated polyenes are formed than by these two latter processes. Sequences of 60 double bonds occur at the dehydrochlorination levels of about 55% that are attained after reaction times of 24 h. These sequences are sterically protected from cross-linking reactions by the presence of longish sequences of unreacted ? CHCl? CH₂? units.     

Formation of conjugated polyenes by chemical and thermal degradation of vinyl chloride copolymers and other vinyl polymers

Raman and infrared spectroscopic studies on the chemical degradation products of two vinyl chloride/vinyl acetate copolymers and a vinyl chloride/vinyl acetate/vinyl alcohol terpolymer, obtained with the system potassium tert-butoxide/N,N-dimethylformamide, show that substantially complete dehydrochlorination occurs. Although the vinyl acetate groups are hydrolysed to the corresponding alcohol these latter largely remain intact and block the extension of the polyene sequences, whose lengths, measured by Raman spectroscopy, are in agreement with values calculated from the initial polymer composition. No degradation other than hydrolysis occurs with vinyl acetate and vinyl alcohol homopolymers or a vinyl alcohol/vinyl acetate copolymer. The thermal degradation of the copolymers containing vinyl chloride, and of two vinyl chloride homopolymers of differing syndiotacticity, at 153°C in N,N-dimethylformamide, gives both dehydrochlorination and elimination of the comonomer units, the latter reaction occurring at least as rapidly as the former. A crosslinked structure, in which there is some steric stabilisation and, hence, conjugated polyene sequence lengths up to about 40, is formed. A maximum sequence length of 8 is found in thermally degraded poly(vinyl acetate), but the conjugated polyenes are too short to be detected in degraded poly(vinyl alcohol) and the vinyl alcohol/vinyl acetate copolymer.     

Resonance Raman spectroscopic studies on dehydrochlorinated stretched poly(vinyl chloride)

Resonance Raman spectroscopy has been used to assess the effect of streching on the formation of conjugated polyenes during the thermal dehydrochlorination of poly(vinyl chloride). Two samples, a commerical mass polymer and a material prepared at ?30°C, have been examined, unplasticised and plasticised, at different draw ratios. Stretching prior to dehydrochlorination leads to an increase in the length of the polyene sequences, the effect being more marked with the low temperature polymer and with increasing draw ratio. Stretching after dehydrochlorination has no significant effect on the polyene distribution. These observations are consistent with the occurrence of chain orientation and alignment during stretching; this reduces the incidence of cross-linking between polyene sequences in adjacent chains and leads to an increase in the average sequence length.     

Thermal dehydrochlorination of poly(vinyl chloride), 2. Transiently and permanently acting structural defects

Samples with an increased content of chloroallyl structures were prepared by chemical degradation of poly(vinyl chloride) (PVC) using potassium tert-butoxide. The samples were thermally dehydrochlorinated in an inert atmosphere, and their dehydrochlorination curves were determined by the continuous potentiometric method. The results show that the chloroallyl structures affect the dehydrochlorination of PVC only transiently. The same conclusion is obtained by an analysis of experimental data of the dehydrochlorination of PVC samples enriched in structures with the chlorine atom bound to the branching site (structures with the chlorine atom at the tertiary carbon atom). A general conclusion is that all structural defects, which facilitate the elimination of hydrogen chloride along the polymer chain and exist in the polymer before its heat stress, play only a transient role in the thermal dehydrochlorination of PVC. These structural defects initiate the splitting-off of hydrogen chloride during the induction period of dehydrochlorination and form conditions for a gradually increasing effect of the basic initiation mechanism, which in the course of dehydrochlorination gives rise to chloroallyl structures. The formation of chloroallyl structures is brought about by a reaction between regular structural PVC units and s-cis-enone, dienone, or polyenone structures. A possible mechanism of this reaction, consisting in an interchain syn-elimination of hydrogen chloride, is suggested.     

Dielectric properties of poly[bis(trifluoroethoxy)phosphazene] and poly[bis(m-methylphenoxy)phosphazene]

Dielectric properties of poly[bis(trifluoroethoxy)phosphazene] (PFEP) and poly[bis(m-methylphenoxy)phosphazene] (PmMPP) were studied in a temperature range from ?195 to 120°C and at several frequencies between 0,1 and 100 kHz. In PFEP, two relaxations, an α relaxation above the glass transition temperature (T_g) and a β relaxation below T_g, which are related to a glass-rubber transition and a local molecular motion of short segments, respectively, were observed. However, PmMPP shows no β relaxation. From the above dielectric results, it was concluded that side groups play an important role in molecular motion of the polyphosphazenes. In PmMPP a γ′ relaxation was found below T_g by immersing the polymers into water. The γ′ relaxation is related to the molecular motin of absorbed water.     

Dual effect of barium on basolateral membrane conductance of frog skin

The effect of Ba²⁺ on basolateral membrane conductance (g _i) in isolated frog skins was analysed. Response patterns were different in tissues with high and low spontaneous intracellular potential. At high (negative) potentials, serosal Ba²⁺ inhibited g _i as is expected of a potent K⁺ channel blocker, whereas in tissues with low potential, g _i remained unchanged or even increased after Ba²⁺. The direction of change in g _i was also dependent on the magnitude of g _i under control conditions. Decrease of g _i was only observed at high g _i in the control period. In contrast, g _i increased if control values of g _i were below 0.5 mS/cm². In tissues with spontaneously low intracellular potential, an inhibitory effect of Ba²⁺ on g _i could be induced by hyperpolarization of the basolateral membrane with transepithelial voltage perturbation. Under these conditions, voltage-dependent, inward rectifying K⁺ channels are activated, which are Ba²⁺-sensitive. Furthermore, hyperpolarization of the basolateral membrane potential (V _i) during Ba²⁺ rapidly decreased g _i. These results suggest that Ba²⁺, in addition to blocking K⁺ channels, activates (presumably unspecific) basolateral membrane channels. This dual effect, which is obvious in tissues with low spontaneous g _i might similarly exist in tissues with high control g _i. Identification, however, is virtually impossible due to the large decrease in potassium conductance.     

Miscibility of poly(vinyl chloride) with ethylene/N,N-dimethylacrylamide copolymers.

The incorporation of N,N-dimethylacrylamide (DMA) as a comonomer in polyethylene yields a material possessing dramatically improved mechanical compatibility with poly(vinyl chloride) (PVC). At increased levels of DMA (25–30 wt.-%) miscibility with PVC is achieved. This behavior is believed to be due to the specific interaction of the tertiary hydrogen of poly(vinyl chloride) (weak “acid” or proton donor) with the disubstituted amide in the ethylene copolymer (weak “base” or proton acceptor). Dynamic mechanical characterization of the ethylene/DMA copolymer (EDMA)/PVC blends reveals separate glass transitions temperatures at DMA levels below 20 wt.-%; they merge into a single T_g when the DMA content reaches a value above 25 wt.-% in the ethylene copolymer. The secondary loss transition for PVC (?40°C) is lowered in temperature and greatly suppressed in magnitude. This is further evidence of molecular miscibility. Mechanical property data obtained on the EDMA/PVC blends are consistent with the foregoing considerations.     

Copolymerization of N-carboxy-α-amino acid anhydrides by organometallic compounds

During thermal degradation of poly(vinyl chloride) in a nitrogen atmosphere polyene sequences of different lengths are formed by dehydrochlorination. The distribution of the sequence lengths in the polymer is approximated by analysis of the electron spectra, and the spectroscopically determined polyene content is compared with the amount of hydrogen chloride split off. With increasing temperature, as well as with increasing time of degradation, the distribution of the polyene sequence lengths shifts towards shorter sequences. In contrast to degraded poly(vinyl chloride) powder, degraded poly(vinyl chloride) foils exhibit a marked shift towards longer sequences. Vinylisobutyl ether as comonomer unit in the polymer chains effects a considerable shift of the sequence lengths distribution towards shorter polyene sequences. The spectroscopically determined polyene content agrees with the amount of liberated hydrogen chloride only a t very low conversions while with increasing dehydrochlorination a growing polyene deficit is found. Therefore, it is to be concluded that polyene sequences are consumed by consecutive reactions in which primarily the longer polyene sequences are involved. A further conclusion that may be drawn from these experiments is that the formation of long polyene sequences takes place very rapidly once the initial step of dehydrochlorination has occurred at a labile site. Apart from the above mentioned consecutive reactions, only the number, not the average length, of the polyene sequences changes substantially with increasing time of degradation.     

Dependence of glass-transition temperature Tg on tacticity of poly(vinyl chloride). A preliminary study by differential scanning calorimetry

The glass-transition temperature, T_g, of poly(vinyl chloride) (PVC) has been found to depend linearly on both the overall isotactic triad content and a small fraction of isotactic triads. It was previously demonstrated, that the latter react with sodium benzenethiolate at temperatures as low as ?30°C, probably because they adopt a definite local conformation. In general, it was believed before that the tacticity effect on T_g occurs only for disubstituted C? C-polymers, the results obtained in this work for PVC would suggest that specific conformations of isotactic triads can also enhance the motion of chain segments in the polymer.     

Local chain configuration dependence of the mechanisms of analogous reactions of PVC, 7. Nucleophilic substitution with potassium 2-ethylhexylthioglycolate and related effects on Tg

The nucleophilic substitution reaction of poly(vinyl chloride) (PVC) with potassium 2-ethylhexylthioglycolate (KEHTG) was performed in a cyclohexanone solution. The evolution of unreacted iso-, hetero- and syndiotactic triad contents with the degree of substitution was observed by means of ¹³C NMR spectroscopy. Due to steric hindrance the fraction of mmr tetrads which react occasionally by the central chlorine of the mr triad instead of the mm is smaller than that found previously for sodium benzenethiolate (NaBT) and sodium thiobenzoate (NaTB), but not nil as in the case of sodium 2-mercaptobenzothiazolate (NaMBT). Correspondingly, the increase in stereoselective character of the reaction as the result of the nucleophile bulkiness is found to approximate to that observed for NaMBT. This conclusion was confirmed on the basis of FTIR results and takes the molecular microstructure-based mechanisms of substitution of PVC, as discussed in previous work, an important step further. From the evolution of T_g with the degree of conversion it follows that T_g varies with the degree of substitution in a similar way to the ratio between the extent to which mmr and rrmr structures intervene in the substitution reaction, thereby providing valuable information regarding the T_g /microstructure relationship as discussed in recent papers.     

Gelatin/dextran intelligent hydrogels for drug delivery: Dual-stimuli-responsive degradation in relation to miscibility in interpenetrating polymer networks

Biodegradable hydrogels consisting of gelatin (Gtn) and dextran (Dex) with an interpenetrating polymer network (IPN) structure were prepared by sequential cross-linking reaction below and above the solgel transition temperature (T_trans) of Gtn, and their enzymatic degradability was examined. IPN-structured hydrogels prepared below T_trans showed to be homogeneous in terms of phase-contrast microscopic observation and small-angle light scattering measurements. In contrast, IPN-structured hydrogels prepared above T_trans showed a phase-separated structure. The water content and compression modulus of the IPN-structured hydrogels were almost constant in contrast to the phase morphology, suggesting the existence of physical chain entanglements between Gtn and Dex networks in the hydrogels prepared below T_trans. Much more bound water was observed in the IPN-structured hydrogel prepared below T_trans, presumably supporting the above suggestion. The IPN-structured hydrogels prepared below T_trans exhibited a specific degradation behavior: degradation of the hydrogels by either α-chymotrypsin or dextranase alone was completely hindered whereas the hydrogel was completely degraded in the presence of both enzymes. Such a specific feature of enzymatic degradation was not observed for the IPN-structured hydrogels prepared above T_trans. It is concluded that the dual-stimuli-responsive degradation can be achieved by enhancing physical chain entanglements of Gtn and Dex networks.     

Enlargement of the glass transition range in compatible polymer blends

Analysis of glass transition temperature (T_g) ranges for compatible polymer blends defined by differential thermal analysis traces for the onset temperature (T_{g, i}) and the final temperature (T_{g, f}) of the glass transition shows the well-known increase in width when compared with the T_g ranges of the blend components. This enlargement of the T_g range is generally attributed to local composition fluctuations; from a comparison of the effects in blends of polystyrenes (PS/PS) of different molecular weights the influence of an increase of the ratio of weight- to number-average molecular weights M?_w/M?_n, because of interactions between the blend components (hetero-interactions) has to be considered, too. Poly(methyl vinyl ether) (PVME)/PS blends are characterized by a peculiar behaviour, the increase in the width of the T_g range being at least three-fold larger than for all other blends studied. This is valid for all blends with a molecular weight of the PS component higher than 10000. Local incompatibilities suggested by enthalpy relaxation and by dynamic mechanical measurements may be responsible for the observed unusual enlargement of the T_g range in the blends with 50–75 wt.-% PS.     

Effect of Thermal Degradation on Glass Transition Temperature of PMMA

Summary: For industrial polymer applications, the structure‐property relation during thermal treatment is of high interest. If a polymer is degraded, its molecular structure will alter, and its mechanical properties are affected as well; thus limiting the usability. The objective of this paper is to highlight the effect of thermal degradation on the glass transition temperature of the remaining polymer. We used commercial atactic poly(methyl methacrylate) (PMMA) as an example. The residue of an oxidatively degraded PMMA was analyzed in terms of its structure‐property relation. A characteristic evolution of the glass transition temperature as a function of degradation time was observed. The glass transition temperature of the degradation product correlates with the molecular weight, monomer content, and tacticity of the remaining polymer. Based on the T_g data, four different degradation domains are discussed in detail. Each period is characterized by a certain trend in T_g due to specialities of the superimposing rate processes.

Glass transition temperature versus normalized weight loss as determined by DDSC measurements.     

Blends of Syndiotactic Polystyrene with SBS Triblock Copolymers

Blending of polystyrene‐block‐polybutadiene‐block‐polystyrene (SBS) triblock copolymers with syndiotactic polystyrene (PSsyn) has been performed in a Brabender mixer above the glass transition temperature of the triblock copolymer but below the melting point of PSsyn. The presence of a large excess of amorphous SBS (at least 70 wt.‐%) as well as of the amorphous PSsyn phase (about 60 wt.‐%, also above its T_g) allowed the easy mixing of the components. In contrast, the presence of unmelted PSsyn crystallites affects both the final morphology of the blend as well as its dynamic behavior. Indeed, according to the dynamic thermomechanical analysis (DMTA) data it is possible to suggest that such solid particles act to reinforce the overall blend structure (as in composite materials filled with inorganic solid particles). Similar behavior is also observed for the SBS copolymer alone below the T_g of the styrene blocks, T_g (PS). This conclusion is supported by both the compatibility of the blends, and the thermal and dynamic thermomechanical behavior at T > T_g (PS) as investigated by means of Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM) and DMTA. The results obtained are tentatively compared with those of similar blends obtained from toluene solution.     

Poly(propylene glycol)-Based Non-Isocyanate Polyurethane Ionenes: Thermal,Morphological and Conductive Properties

The synthesis and characterization of a series of polyurethane ionenes using a non-isocyanate approach is disclosed. Imidazole-capped, urethane-containing prepolymers are prepared by first reacting carbonyl diimidazole (CDI) with several poly(propylene glycol) (PPG) diols with variable molecular weight, followed by subsequent reaction with 3-aminopropylimidazole (API). Polymerization with 1,4-dibromomethylbenzene followed by anion exchange resulted in the desired polyurethane ionenes bearing the [NTf₂] counteranion as a series of viscous liquids. NMR and FTIR spectroscopy are used to characterize the intermediates and final ionenes, including molecular weight determination by end-group analysis. A single glass transition temperature (T_g), as determined by differential scanning calorimetry (DSC), is observed for each ionene (−38 to −64 °C) with the T_g decreasing with increasing PPG molecular weight. Thermogravimetric analysis (TGA) indicated a two-step decomposition for each ionene, with the first being degradation of the PPG segment, followed by the urethane/ionic segment. Microphase separation is observed from x-ray scattering profiles with Bragg distances that increased with increasing PPG molecular weight. Ionic conductivity is found to be inversely dependent upon DSC T_g at lower temperatures (RT and below); however, at higher temperatures, conductivity appears to be more dependent upon the ability of ionic aggregates caused by phase separation to interact.     

Facile Monitoring of Fullerene Crystallization in Polymer Solar Cell Blends by UV–vis Spectroscopy

A simple technique is demonstrated based on UV–vis spectroscopy that permits the onset of fullerene crystallization to be monitored in polymer:fullerene bulk‐heterojunction blends. The growth of fullerene crystals only occurs during thermal annealing above the glass transition temperature (T_g) of the blend. Light scattering due to the presence of micrometer‐sized fullerene crystals reduces the apparent transmittance, which can serve as a clear indicator for the upper limit of the T_g. This principle is used to deduce this critical parameter for two high‐performance photovoltaic blends that are based on a thiophene–quinoxaline and a thiophene–isoindigo copoly­mer, respectively. High T_gs of 115 and 145 °C promise good thermal stability.

     

Phase morphologies in block copolymers of polystyrene and columnar liquid crystalline polydiethylsiloxane

Well-defined diblock copolymers of polystyrene (PS) and polydiethylsiloxane (PDES) were synthesized by sequential anionic polymerization. By means of differential scanning calorimetry (DSC), the PDES blocks were shown to exist in the columnar mesophase at ambient temperatures. PS-b-PDES block copolymers with a PS content > 21 wt.-% build lamellar structures, below this percentage PS cylinders were found that are arranged in sheets, forming a (somewhat irregular) overall lamellar structure. Both the low PS content at which the transition from lamellar to cylindrical structures takes place, as well as the peculiar organization of the PS cylinders in the latter were ascribed to the mesomorphic nature of the PDES block in combination with the fact that the glass transition temperature of PS (T_g ≈ 100°C) is much higher than the isotropization temperature of the PDES block (T_i 〈ca. 50°C).     

Physical aging of poly(ether sulfone) from enthalpy relaxation measurements

Enthalpy relaxation due to physical aging of hydroxyl-terminated poly(ether sulfone)s (PES) with various molecular weights was studied by DSC at three aging temperatures between T_g ? 45°C and T_g ? 10°C. The experimental results revealed that both the average enthalpy relaxation time 〈τ〉 and limiting enthalpy relaxation ΔH_∞ decrease slightly with increasing molecular weight for PESs aged at the same temperature intervals T_g ? T_a (where T_a < T_g). This behavior is not quite consistent with the enthalpy relaxation process of polystyrene. Agrawal reported that the relaxation spectra for polystyrenes with different molecular weights could be superimposed when T_g ? T_a was the same. The experimental results of the present work suggest that the motion of segments is affected by intermolecular hydrogen bonding through the ? OH groups of the PES chain ends.     

Taktizität und eigenschaften anionisch polymerisierter poly-α-methylstyrole

α-Methylstyrene was polymerized with sodium naphthalene in tetrahydrofuran (THF) at ?78°C. At constant ratio monomer/solvent 1:10 (vol/vol), the fraction T_i of isotactic triads increases with increasing ratio monomer/initiator (increasing mol. wt. of polymer) from T_i = 0.35 (M = 2700) to T_i = 0.56 (M > 2·10⁵) whereas the fraction T_h of heterotactic triads decreases from T_h = 0.22 (M = 2700) to T_h = 0.10 (M > 20,000). The fraction of syndiotactic triads T_s = 0.41 is constant up to M < 5·10⁴ and then decreases up to T_s = 0.33 (M > 2.10⁵). Replacing tetrhydrofuran partly by hexane respectively using tetrahydropyran (THP) as a solvent for the polymerisation, T_i increases, particularly in the case of low molelular weight products. Samples with M < 3000 measured by vapour pressure osmometry showed a tendency towards selfassociation in bezene, cyclohexane, and chloroform, respectively. It is probablly a unimer/N-mer type of association. The viscosity/molecular weight relationship [η] = KM shows anomalies in terahydrofuran, toluene, and cyclohexane below a molecular weight of 45,000. In the same range, the slope of the function T_g = f(1/M_w) also changes. The glass temperatures T_g of the polymers were found to increase by polymerization in mixtures of tetrahydrofuran and hexane.     

Ester-carbonate interchange reaction in the solid state: synthesis of poly(ester-carbonate)

Poly(ethylene terephthalate-co-bisphenol A carbonate) was synthesized by ester-carbonate interchange reaction in the solid state. The polymerization reaction was carried out by heating two chemically distinct oligomers, namely, poly(aryl carbonate) and poly(ethylene terephthalate) in the temperature range of 180–230°C, during which period both chain extension as well as interchange reactions occurred. A copolymer of η_inh = 0.84 dL/g was obtained. With the progress of the interchange reaction the T_g of the copolymer slowly increases and reaches a maximum value of 102°C. The oligomer mixture, which exhibits initially two distinct T_m's, shows a single T_m at the end of the reaction. Formation of copoly(aryl ester-carbonate)s by an ester-carbonate interchange reaction was also found in the solid state at temperatures below 230°C. High molecular weight poly(aryl ester-carbonate)s with a T_g = 163°C and η_inh = 0.52 dL/g were prepared from the corresponding low molecular weight oligomers of poly(aryl ester) and poly(aryl carbonate)s.