Effects of Spacer Length and Terminal Group on the Crystallization and Morphology of Biscarbamates: A Longer Spacer Does Not Reduce the Melting Temperature

The effects of alkyl side chain and spacer lengths and the type of terminal group on the morphology and crystallization of a homologous series of biscarbamates (model compounds for polyurethanes) were investigated. Biscarbamates were synthesized with alkyl side chains of various lengths ranging from C₄ to C₁₈ and an alkyl spacer group with 12 CH₂ units (C₁₂ spacer) between the two hydrogen bonding motifs. The crystallization and morphological features are compared with the previously studied biscarbamates with a C₆ spacer. As a token example, we also studied a biscarbamate molecule in which the terminal methyl group was replaced by a phenyl group. We stress four important conclusions of the study: (1) A number of studies in the literature found that the longer alkyl spacers reduced the thermal transition temperatures of the molecules, and such behavior was attributed to an increase in the flexibility of the alkyl spacer. However, the results of the present study are to the contrary. With the biscarbamates studied here, the hydrogen-bonding groups on both sides of the C₁₂ spacer act as “anchors”, and the longer spacer does not reduce the melting temperatures compared with those with the C₆ spacer. (2) The melt viscosity measurements show shear-thinning behavior, which has been mostly observed with polysaccharides and hydrogen-bonded polymers. (3) Avrami analysis shows a two-stage crystallization, which is not commonly observed in organic small molecule systems. (4) The phenyl end group does not add another self-assembly code in terms of π-stacking but acts as a defect. While formation of crystals was observed for biscarbamates with short alkyl side chains with a C₆ spacer, an increase in spacer length to C₁₂ induces spherulitic morphology. Although the overall sizes of the spherulites are the same for both spacers, the rate of spherulite growth was higher and the crystallization rate was lower with the C₁₂ spacer compared with the C₆ spacer. In contrast with the biscarbamates containing C₆ spacer previously studied, we find that among the biscarbamates with 12 CH₂ units in the spacer the C₁₂–C₁₂ molecule shows the minimum spherulite size, spherulite growth rate, and rate of crystallization. The infrared frequency shifts of the N–H group due to hydrogen bonding were used to calculate the N···O hydrogen-bonding distance and found to be close to the value observed in the crystal structure of the biscarbamate with a C₆ spacer and C₁₀ alkyl side chain

Synthesis and Properties of Bis(3,5-dimethyl-4-amino phenyl)-4′-methoxy phenyl Methane Based Polymerization of Monomeric Reactants Type Polyimide and Its Composites

Polymerization of Monomeric Reactants (PMR) type of polyimide was prepared using bis­(3,5-dimethyl-4-amino phenyl)-4′-methoxy phenyl methane and characterized by elemental analysis, FT-IR, ¹H NMR, and ¹³C NMR spectral techniques. Differential scanning calorimetry (DSC) analysis shows that melting and curing take place in the temperature range of 215 to 230 °C and 265 to 315 °C respectively. The uncured and cured prepolymer undergo two step (335 and 583 °C) and single step (537 °C) decomposition respectively. The <i>T</i>_10% and char yield at 800 °C were found to be 465 °C and 46 wt %, respectively. FT-IR and thermogravimetric analysis (TGA) of the heat treated resin powder and prepolymer reveal that imidization takes place in the temperature range 250–275 °C and cross-linking occurs between 275 and 325 °C. The prepolymer was found to undergo photocross-linking. The flexural strength, flexural modulus, and interlaminar shear strength of carbon fiber (T300) reinforced BA4MPM-PMR type composite are 447 MPa, 34 GPa, and 42 MPa, respectively