Article pubs.acs.org/Macromolecules
Trimerization Reaction Kinetics and Tg Depression of Polycyanurate under Nanoconfinement Evelyn Lopez and Sindee L. Simon* Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409-3121, United States ABSTRACT: Trimerization of a mixture containing a mono- and difunctional cyanate ester is investigated under the nanoporous confinement of silanized hydrophobic controlled pore glass using differential scanning calorimetry. The trimerization reaction of the nanoconfined monomer mixture is accelerated relative to the bulk by as much as 12 times in 8 nm pores, but this acceleration is less than half that observed for nanoconfinement of the individual monomers. The absolute reaction rate of the monomer mixture lies between those of the individual species, being slower than the monocyanate ester and faster than the dicyanate ester. The results are consistent with the hypothesis that the reaction acceleration is due to monomer ordering or layering at the pore surface, leading to a local concentration of reactive groups higher than in the bulk. In addition to the influence of nanoconfinement on trimerization kinetics, the molecular weight and glass transition temperature (Tg) of the polycyanurate formed in the nanopores are investigated. The molecular weight decreases approximately 20% for synthesis in the smallest 8 nm pores relative to the bulk value of 5200 g/mol. Upon extraction from the pores, the polymer Tg is 5−9 K higher than in the bulk. However, in the 8 nm diameter pores, a Tg depression of 44 K is observed relative to the value of the material after extraction from the pores. This depression lies between the values previously observed for the products of the individual cyanate esters which formed a low molecular weight trimer and a cross-linked polymer network. A secondary Tg, associated with a less mobile layer at the pore wall, is 26−40 K above the primary value. The implication is that the origin of confinement effects on reactivity and Tg differ, with changes in reactivity in this system arising from surface layering or ordering and Tg depressions arising from intrinsic size effects.
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INTRODUCTION Nanoconfinement is known to influence material properties, including changes in reactivity. Previous work in our laboratory has focused on changes in reactivity in nanopores for the free radical polymerization of methyl methacrylate and the trimerization reaction of cyanate esters. In the case of methyl methacrylate, the higher reactivity under nanoconfinement was explained primarily by a decrease in diffusivity of nanoconfined chains which results in a decrease in the rate of termination and earlier autoacceleration.1,2 On the other hand, the origin of the acceleration in the trimerization reaction is not as clear. Acceleration of the reaction rate is observed for both a monofunctional monomer that forms a low molecular weight trimer and for a difunctional monomer that forms a crosslinked polycyanurate network; the reaction acceleration is similar for the two monomers although it does depend slightly on reaction conditions.3−7 The acceleration may be attributable to layering of functional groups near the pore wall, as suggested by simulations of Malvaldi et al.8 where such ordering led to an increase in the local concentration of reactive groups near the wall. Recently, Yancey and Vyazovkin found that reactivity decreased under nanopore confinement for a solid-state reaction in which ordering was presumably reduced for the confined material in comparison to the bulk,9 whereas reactivity increased for a liquid state trimerization of molten dicyanamide salts under nanopore confinement.10 In order to test how ordering or layering may affect reactivity in cyanate esters under © 2015 American Chemical Society
nanopore confinement, we attempt to disrupt layering by polymerizing a monomer mixture containing two monomers having different sizes and structures. The results are compared to prior studies of the individual species.3−7 In addition to changes in reactivity, the glass transition temperature (Tg) is also influenced by nanoconfinement with observed behavior depending on confinement geometry, surface/interface chemistry, measuring technique, and the glass former itself.4,5,11−17 Decreases, increases, and unchanged Tgs are observed under confinement, presumably due to competition between intrinsic size effects3,6,12,18 and mobility changes at free surfaces and interfaces.19−24 Previous work on cyanurates under nanopore confinement shows that T g decreases with decreasing pore size and the observed depression for a cross-linked polycyanurate network is greater than that of a low molecular weight cyanurate trimer.4,5 In addition, for the cross-linked polycyanurate, the Tg depression increases with conversion as the network develops.5 In this work, we extend the studies of nanoconfined cyanurates to include the un-cross-linked polycyanurate synthesized from a mixture containing a mono- and difunctional cyanate ester. Received: January 25, 2015 Revised: June 3, 2015 Published: June 29, 2015 4692
DOI: 10.1021/acs.macromol.5b00167 Macromolecules 2015, 48, 4692−4701
Article
Macromolecules
Figure 1. Chemical structure of (a) monofunctional 4-cumylphenol cyanate ester, (b) difunctional bisphenol M dicyanate ester, and (c) schematic of cyanurate trimerization. Also shown on the right are the 3-D models for the monocyanate and dicyanate ester monomers as generated by Chem3D Pro 14.0.
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from 8.1 to 111.1 nm. The bulk density of the particles ranges from 302 to 526 g/L, as provided by the manufacturer. The specifications of the controlled pore glasses listed in Table 1 are also provided by the
EXPERIMENTAL SECTION
Materials. A monofunctional cyanate ester, 4-cumylphenol cyanate ester (Oakwood products), and a difunctional cyanate ester, bisphenol M dicyanate ester (BMDC, trade name RTX-84921 from Hi-Tek Polymers, Louisville, KY), are used in this study. Their chemical structures and 3-D conformations are shown in Figure 1, and the cyanurate reaction product arising from trimerization of three cyanate ester functional groups is shown in Figure 1c. The molecular weights of the mono- and difunctional monomers are 237 and 396 g/mol and their densities at 25 °C are 1.10 and 1.14 g/cm3, respectively. The monofunctional monomer is in its liquid state at room temperature, whereas the difunctional monomer is a metastable liquid, and thus, before mixing, the bisphenol M dicyanate was heated to 100 °C for 3 min to remove any crystals (Tm = 60 °C) that formed during storage in the freezer. The monomer mixture is composed of 44 wt % monofunctional cyanate ester and 56 wt % difunctional cyanate ester, resulting in 40 mol % of cyanate groups being monofunctional. The liquid monocyanate ester and bisphenol M dicyanate ester monomers were mixed at room temperature for 30 min, and the mixture was degassed under vacuum prior to reaction, following previous procedure in our laboratory.25 The monomer mixture has a density of 1.15 ± 0.02 g/ cm3 and, within error, is the same as that expected for an ideal mixture of the individual species. The mixture composition was chosen so that the resulting reaction product would be an un-cross-linked polycyanurate based on our previous work,25 and in fact, the molar mass of the polycyanurate is found to be approximately 5000 g/mol (see later for exact numbers for various syntheses). The nanoconfined medium used is controlled pore glass (CPG, a borosilicate glass from Millipore, Billerica, MA) with pore sizes ranging
Table 1. Specification of Controlled Pore Glasses As Provided by the Manufacturer product name
mean pore diama (nm)
pore diam distributionb (%)
specific pore vola (cm3/g)
specific surf. areac (m/g)
CPG00080 CPG00130 CPG00500 CPG01000
8.1 13.0 54.8 111.1
9.0 7.4 2.5 4.2
0.49 0.68 1.18 1.37
197.0 130.0 49.5 27.4
a
Determined by the mercury intrusion method. bAnalyzed by the ultrasonic sieving method. cMeasured by the nitrogen adsorption method. manufacturer. To eliminate the effect of hydroxyl groups on the CPG surfaces, the controlled pore glasses were derivatized with hexamethyldisilazane to convert the surface hydroxyl groups to trimethylsilyl groups following the work of Jackson and McKenna.26 The silanized CPGs are stored in a desiccator before use to minimize moisture absorption. DSC Measurements. A PerkinElmer Pyris 1 differential scanning calorimeter, equipped with a PerkinElmer Intracooler maintained at −55 °C, was used for calorimetric measurements. Controlled pore glasses with weights ranging from 2.2 to 7.1 mg were loaded into 20 μL PerkinElmer hermetic pans followed by the monomer mixture with 4693
DOI: 10.1021/acs.macromol.5b00167 Macromolecules 2015, 48, 4692−4701
Article
Macromolecules
Fourier Transform Infrared Spectroscopy (FTIR). A Thermo Nicolet Nexus 470 Fourier transform infrared spectrometer (FTIR) was used to study the chemical structures pre- and postreaction. The monomer spectrum was collected with the sample placed between two polished NaCl salt plates. The spectra for the polymer were collected after placing a drop of polycyanurate/THF solution on the salt plate and allowing the solvent to evaporate.
a sample size ranging from 2.3 to 4.0 mg. Samples of the bulk, without any CPG, were also loaded into DSC pans with weights ranging from 2.1 to 4.2 mg. The samples were prepared, and the DSC pans were sealed under a nitrogen environment. The pan was then held at 80 °C for 10 min, followed by an additional 10 min at 100 °C to ensure complete imbibement.3 Negligible conversion (
