Atactic PMMA Research Articles

Polymerization of methyl methacrylate with alane complexes of bivalent lanthanidocenes

Alane complexes of some bivalent ytterbocenes, L 2YbAlH 3·NEt 3 (L=C 5Me 5 ( IV), 1-SiMe 3Ind ( V), 9-SiMe 3Flu ( VI)) were found to provide stereospecific polymerization of methyl methacrylate. The apparent activities of initiators IV– VI were estimated to be a few times greater than those of their monometallic analogues L 2Yb·THF ( I– III). The polymerization was shown to proceed via the formation of a catalytically active intermediate, L 2YbAlH 2[OC(OMe)C(Me)CH 2C(Me) 2(CO 2Me)] which was proposed to exist in various forms. At low temperatures the alane fragment did not disrupt the stereocontrol mechanism originating from the metallocene template. The catalysts IV– VI produced an atactic pMMA at 0°C.

Influence of tacticity of poly(methyl methacrylate) on the compatibility with poly(vinyl phenol)

Isotactic, atactic, and syndiotactic poly(methyl methacrylate) (PMMA) were mixed with poly(vinyl phenol) (PVPh) separately in tetrahydrofuran to make three polymer blend systems. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to study the miscibility of these blends. Isotactic PMMA was found to be more miscible with PVPh than atactic or syndiotactic PMMA.

Influence of tacticity of poly(methyl methacrylate) on the compatibility with poly(vinyl phenol)

Isotactic, atactic, and syndiotactic poly(methyl methacrylate) (PMMA) were mixed with poly(vinyl phenol) (PVPh) separately in tetrahydrofuran to make three polymer blend systems. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to study the miscibility of these blends. Isotactic PMMA was found to be more miscible with PVPh than atactic or syndiotactic PMMA. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1773–1780, 1997

Structural Heterogeneity in Poly(methyl methacrylate) Glasses of Different Tacticity Studied by Thermally Stimulated Current Thermal Sampling Techniques

The thermally stimulated current thermal sampling (TSC-TS) technique was used to study the cooperative glass transition like relaxations in different tacticity poly(methyl methacrylates) (PMMAs) including iso- and syndiotactic forms. We use the high sensitivity of the TSC-TS method to study these weak and sometimes overlapping relaxations at a frequency of about 10 -3 Hz where one can resolve cooperative relaxations well below the main glass transition (Tg), even if the species are a minor fraction of the overall relaxing species. The magnitudes of the values of E a are compared with a theoretical prediction as a function of temperature, showing that the low-temperature tail of the glass transition in conventional atactic (e.g., 45% syndiotriads) extends down to 15 °C, while in our highest syndiocontent PMMA (80% syndiotriads), the glass-transition-like relaxations only extend down to about 80 °C. Results taken from the literature also substantiate the results obtained here for atactic PMMA. The results are contrasted to those for bisphenol-A polycarbonate and other polymers and suggest that compositional heterogeneity, more specifically the presence of low T g pockets of predominantly isotactic sequences, contributes to the broad glass transition extending almost 90 °C below the main glass transition in atactic PMMA.

Influence of Tacticity on Polymer Surfaces Studiedby ToF-SIMS

Poly(methylmethacrylate) (PMMA) and polystyrene (PS) thin films of different tacticities have been analysed by time-of-flight (ToF) SIMS in order to look for a possible influence of the stereoregularity on their fingerprint spectra (m/z⩽200). Because, at first glance, the spectra of samples with different tacticities seemed rather similar, the intensity ratios between secondary ions from main polymer chain and those from pendant group were evaluated. By this procedure, it was found that isotactic PMMA can be clearly distinguished from syndiotactic and atactic PMMA. A lower concentration of the methyl-methacrylate pendant group was detected at the surface of the isotactic sample. In the case of PS, more phenyl groups were found to be present at the surface of isotactic PS and an empirical criterion is proposed to differentiate between atactic and isotactic PS. © 1997 by John Wiley & Sons, Ltd.

Molecular Conformation and Application of Stereoregular PMMA Langmuir-Blodgett Films

Molecular conformations of stereoregular poly(methyl methacrylate) (PMMA) monolayers have been investigated by scanning probe microscopes. Isotactic and syndiotactic PMMAs were found to have right and left hand helical structures, respectively. On the contrary atactic PMMA showed rather random arrangement of the chains. It has been demonstrated that the PMMA Langmuir-Blodgett(LB) films can be utilized to form nanoscale patterns down to 50 nm and to form a geodesic lens. It has also been manifested that the quantum efficiency of a polymer electroluminescent device can be significantly enhanced by inserting the PMMA LB films between the emitting layer and the cathode. All the applications utilize the unique characteristics of the LB films to form thin and uniform films in the molecular level.

Spin Label Study of the Factors Affecting Molecular Motion of Poly(methyl methacrylate)

The electron spin resonance (ESR) line shape of nitroxide radical labels attached to poly(methyl methacrylate) (PMMA) chains was studied as a function of temperature. In order to clarify various factors affecting molecular motions of PMMA chains, molecular mobility of a spin-labeled PMMA chain (L-chain) in another PMMA (M-chain) matrix was estimated by the ESR method. Isotactic and atactic PMMA having narrow molecular weight distribution were used as L- and M-chains. Strong dependencies of molecular mobility on molecular weights (ML and MM) of the chains and tacticity of M-chains were found. For example, the transition temperature, T50G at which the extrema separation width due to anisotropic hyperfine splitting drops sharply increases with increasing molecular weight of M-chains. The correlation times at the transition temperatures, calculated from the ESR spectra are compared with those in so-called relaxation map and the nitroxide rotational motion is concluded to strongly correlate with micro-Brownian motion of the PMMA chains. It was also found that the correlation times estimated by the ESR method is proportional to ML0.5 and MM when MM<2Me, and independent of MM when MM>2Me. Here, Me is the molecular Me weight between entanglements. These expetimental facts are reflected on Stokes-Einstein and reptation types of diffusions of L-chains.

Fluorescence Depolarization Study of Local Motions in Polymers at the ϑ Temperature

The mean relaxation times, Tm's, of four different polymers measured by the fluorescence depolarization method were compared at their ϑ temperatures. These polymers, i.e., cis-polyisoprene (cis-PI), polystyrene (PS), poly(α-methylstyrene) (PαMS), and syndiotactic poly(methyl methacrylate) (s-PMMA), were labeled with anthracene in the middle of the main chain by living anionic polymerization. The ϑ temperature is a standard point without an excluded volume effect, and the ϑ temperature of these four polymers are almost the same, 34−35 °C. Therefore, the comparison of the Tm's at the ϑ temperature enables us to clarify the relationship between the chain local motion and the chemical structure. The order of the reduced relaxation times, Tm/η, was Tm/η(cis-PI) < Tm/η(PS) < Tm/η(PαMS) < Tm/η(s-PMMA), and the order of activation energies, E*, was E*(cis-PI) < E*(PS) < E*(PαMS) < E*(s-PMMA). From a molecular standpoint, the relaxation times depend on both the substituents attached to the main chain and the stereoregularity of the polymer chains. In consideration of the stereoregularity of the polymer chains, the relaxation time of atactic PαMS is estimated to be not much different from the relaxation time of atactic PMMA.

Mean-Square Optical Anisotropy of Isotactic Oligo- and Poly(methyl methacrylate)s in Dilute Solution

The mean-square optical anisotropy was determined from anisotropic light-scattering measurements for 10 samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f r ≃ 0.01, in the range of weight-average molecular weight from 3.58 x 10 2 (trimer) to 1.07 x 10 4 , and also for methyl isobutyrate (the monomer of PMMA), in acetonitrile at 28.0 °C (Θ). A comparison is made of the present data with the helical wormlike (HW) chain theory with the values of the model parameters previously determined from the mean-square radius of gyration and with the local polarizability tensor properly assigned to the repeat unit of the chain in the same manner as in the previous study of atactic (a-) PMMA, i.e., evaluated with the same bond polarizabilities. It is shown that the HW theoretical values may reproduce quantitatively the experimental results in the range of weight-average degree of polymerization x w ≥ 6. The disagreement between theory and experiment for x w ≤5 may probably be due to effects of chain ends. A comparison is also made of the present results with the previous ones for a-PMMA, leading to the conclusion that the HW theory may explain the f r dependence of as well as of , the scattering function, and steady-state transport coefficients.

Transport Coefficients of Isotactic Oligo- and Poly(methyl methacrylate)s in Dilute Solution

The intrinsic viscosity [η] was determined for 25 samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMMA), each with the fraction of racemic diads f r ≃ 0.01, in the range of weight-average molecular weight M w from 3.58 x 10 2 (trimer) to 1.71 x 10 6 in acetonitrile at 28.0°C (Θ). The translational diffusion coefficient D was also determined from dynamic light scattering measurements for 12 of them in the range of M w from 6.58 x 10 2 (hexamer) to 9.46 x 10 5 under the same solvent condition. It is found that [η] is proportional to M w 1/2 for M w ≥ 5 x 10 4 and its deviation from this asymptotic behavior is small even for smaller M w , while D is inversely proportional to M w 1/2 except for M w ≤ 2 x 10 3 . Such apparent Gaussian behavior of [η] and D over a wide range of M w is the result expected from that previously obtained for the mean-square radius of gyration (S 2 ). From an analysis of these transport coefficients on the basis of the helical wormlike (HW) chain model, it is shown that the above M w dependences of [η] and D may be well explained by the HW theories with the values of the model parameters consistent with those previously determined from (S 2 ). A comparison is also made of the present results for [η] and D for i-PMMA with the previous ones for atactic (a-) PMMA with f r = 0.79. This leads to the confirmation of the previous conclusion derived from (S 2 ) concerning the f r dependence of the chain stiffness and local chain conformation of PMMA. That is, the i-PMMA chain is of weaker helical nature than the chain that retains rather large and clearly distinguishable helical portions in dilute solution.

Modelling the structure of a polymer glass poly(methylmethacrylate) through neutron scattering experiments

Determination of the local structure of a polymer glass by scattering methods is complex due to the number of spatial and orientational correlations, both from within the polymer chain (intrachain) and between neighbouring chains (interchain), from which the scattering arises. Recently considerable advances have been made in the structural analysis of relatively simple polymers such as poly(ethylene) through the use of broad Q neutron scattering data tightly coupled to atomistic modelling procedures. This paper presents the results of an investigation into the use of these procedures for the analysis of the local structure of a-PMMA which is chemically more complex with a much greater number of intrachain structural parameters. We have utilised high quality neutron scattering data obtained using SANDALS at ISIS coupled with computer models representing both the single chain and bulk polymer system. Several different modelling approaches have been explored which encompass such techniques as Reverse Monte Carlo refinement and energy minimisation and their relative merits and successes are discussed. These different approaches highlight structural parameters which any realistic model of glassy atactic PMMA must replicate.

Scattering Function of Isotactic Oligo- and Poly(methyl methacrylate)s in Dilute Solution

The scattering function was determined for four samples of isotactic oligo- and poly(methyl methacrylate)s (i-PMA), each with the fraction of racemic diads f r ≃0.01, in the range of molecular weight M n from 1.01×10 3 to 9.87×10 4 in acetonitrile at 28°C (θ) for tbe magnitude k of the scattering vector smaller than 1 A -1 by the use of a point-focusing SAXS camera. The Kratky function F s as a function of k increases monotonically with increasing k irrespective of the values of Mn and does not exhibit a maximum followed by a minimum, as observed for atactic (a-) or syndiotactic (s-) PMMA

Scattering Function of Syndiotactic Poly(methyl methacrylate) in Dilute Solution

The scattering function was measured for a syndiotactic poly(methyl methacrylate) (s-PMMA) sample with the fraction of racemic diads f r = 0.92 and molecular rveight M w = 3.76×10 4 in acetonitrile at 44.0°C (θ) for the magnitude k of the scattering vector smaller than 1 A -1 by the use of the same point-focusing SAXS camera as used in a previous SAXS study of atactic (a-)PMMA samples with f r = 0.19. Supplementary measurements were also carried out on the same s-PMMA sample in benzene at 25.0°C

Structural study of a poly(methylmethacrylate) Langmuir–Blodgett film on a graphite surface by scanning tunneling microscope

In this study, molecular configurations of three different stereoregular poly(methylmethacrylate) (PMMA) (isotactic, atactic, and syndiotactic PMMA) monolayers, which were transferred onto graphite substrates by the Langmuir–Blodgett (LB) technique, were investigated using scanning tunneling microscope (STM). STM images of monolayer films taken in air in the constant height mode showed various types of film structures. First, polymer films in their different forms always showed long-range ordered chain structures. Second, these polymer chains were generally observed to arrange themselves in parallel chains with no backbone crossovers. However, the packing density of polymer chains on the surface varied with change of scanning area presumably due to the nonuniformity of the LB film and the effect of thermal fluctuation of polymer chains on the film. Third, a monolayer film of isotactic PMMA film showed the coexistence of helical and linear chains in most of the film areas suggesting that the structural transition from the linear chains to 51 single stranded helices occurs at the surface pressure of 8 mN/m.

Segmental motion of stereoregular poly(methyl methacrylates) in a homologous series of n‐alkyl acetate solvents by nmr relaxation measurements

AbstractThe 13C spin‐lattice relaxation times, T1s, of both stereoregular poly(methyl methacrylates) (PMMA) and homologous series of n‐alkyl acetate solvents in solution have been measured at 40°C. It was observed that the motion of polar side‐chains was highly affected by the degree of interaction with solvent molecules. The stronger polymer‐solvent interactions in predominantly syndiotactic PMMA solutions than in isotactic PMMA solutions were shown from the T1 values for the carbonyl, methoxy, and quaternary carbons in polymer segments and also from the carbonyl T1s of solvent molecules. By relating the solvent dependences of the T1 data in syndiotactic polymer solutions to that of the known dissolution rate data of atactic PMMA, it was found that the solvent dependences of the T1 values of those carbon groups in which the polymer‐solvent interaction is not significant, e.g., methylene and α‐methyl carbon groups, were consistent with the solvent dependence of the dissolution rate of polymer. This result suggested that the dissolution of polymer is mainly governed not by the sorption process related to the polymer‐solvent interaction but by the transport process related to the local motions of polymer segments and solvent molecules.

Poly(ethylene oxide)-poly(methyl methacrylate) interactions in polymer blends: an infra-red study

Following our previous work in this field it is shown that the vibrational infra-red spectrum indicates that polyoxyethylene, usually in the most stable helical structure (H), transforms into atrans planar structure (T) when in contact with syndiotactic poly(methyl methacrylate) (PMMA). The conversion of solid H to solid T decreases gradually from syndiotactic PMMA to atactic PMMA to isotactic PMMA. A comprehensive interpretation is proposed in order to reconcile the detailed spectroscopic data reported here and the overall morphological descriptions reported by other authors.

Ordered structures of stereoregular poly(methyl meth‐acrylates) in solutions studied by spectroscopic methods

AbstractThree groups of problems concerning ordered structures in solutions of stereoregular poly(methyl meth‐acrylates) (PMMA) were studied by spectroscopic methods: (i) Self‐aggregation of syndiotactic (s) PMMA in solution was followed by means of NMR and IR spectroscopy; it has been established that double helices are formed during the initial stage of aggregation, and activation and equilibrium thermodynamic parameters (ΔH, ΔS) which characterize the transition coil–double helix in s‐PMMA were determined. (ii) NMR spectroscopy has demonstrated that association in solutions of atactic PMMA proceeds via interactions of stereocomplex type (interaction between m‐diads and r‐tetrads). (iii) It has been found that 13C CP/MAS NMR spectra of crystalline forms of PMMA (prepared from aggregated solutions by solvent evaporation) conform to the double‐helix structure of these systems.

New NMR Methods that Probe the Structure of Glasses

AbstractTwo-dimensional NMR methods allow a determination of the relative orientations of nearby molecules in polycrystalline and noncrystalline solids. We describe these methods and present applications to the determination of the crystal structures of two forms of polycrystalline methanol and to the investigation of short range molecular orientational ordering in methanol glass and in atactic PMMA.

The effect of molecular properties on the mechanical behaviour of PMMA—IV. The role of chain entanglements in fracture processes

The fracture-toughness—entanglement correlation was shown to be a more fundamental concept than the fracture-toughness—glass transition temperature or the fracture-toughness—solubility-parameter correlations in predicting the effect of molecular weight, molecular weight distribution, stereoregularity and temperature on the fracture behaviour of poly(methyl methacrylate) (PMMA). Morphological study of the fracture surface by scanning electron microscopy (SEM) of high molecular weight PMMA in good and bad solvents and blends of isotactic PMMA and low molecular weight atactic PMMA supports the relationship between molecular entanglements and fracture behaviour. The entanglement network at the crack tip determines whether failure occurs in solvents by rapid stress-cracking or stress-crazing with long-craze growth.

An infrared study of azobenzene photoisomerization in a polymer matrix

A technique involving in situ Fourier transform infrared spectroscopy in the reflection-absorbance mode (FTIR-RA) was used to study photoisomerization and the thermal back reaction of a solid solution of trans-azobenzene in a poly(methyl methacrylate) (PMMA) matrix. The FTIR-RA method, although inherently less sensitive than UV spectrophotometry, affords better resolution of specific isomer absorbances and permits simultaneous observation of changes in the matrix and chromophore vibrational bands. A change in the orientation of the acrylate side groups in isotactic sequences of the atactic PMMA matrix was observed to coincide with photoisomerization of the dissolved azobenzene. This orientational change is a direct confirmation of the matrix effect on isomerization reactions postulated by other researchers. The frequencies of the most intense vibrational absorption bands in the thermally unstable cis isomer were determined. With this information, the relative quantum yields φ t→c φ c→t for photoisomerization of azobenzene and the half-life for thermal cis→trans isomerization were obtained and compared with literature values.