PHÂN TÍCH NHIỆT PMMASILICA GEL TIẾNG ANH

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/226303404 Thermal analysis of PMMA/gel silica glass composites Article in Journal of Sol-Gel Science and Technology · January 1996 DOI: 10.1007/BF00401038 CITATIONS READS 30 41 Some of the authors of this publication are also working on these related projects: Investigating certain types of direct Brain-Machine Interaction View project All content following this page was uploaded by Fotini Pallikari on 14 September 2017 The user has requested enhancement of the downloaded file Journal of Sol-Gel Science and Technology 7,203-209 (1996) @ 1996 Kluwer Academic Publishers Manufactured in The Netherlands Thermal Analysis of PMMA/Gel Silica Glass Composites FOTINI PALLIKARI-VIRAS Physics Department, University ofAthens, Zografos, Panepistimiopolis, Athens 157 84, Greece XIAOCHUN LI AND ‘IERENCE A KING Department of Physics and Astronomy, Schuster Laboratory, University of Manchestel; Manchester Ml3 9PL, UK Received June 13, 1995; Accepted February 12, 1996 Abstract Thermal analysis of poly-methylmethacrylate (PMMA) impregnated porous gel silica glasses confirms that the PMMA chains form hydrogen bonds with the pore surface silanol groups The adopted conditions for the insitu polymerisation result in about 4% of residual monomers trapped in the polymer, most of them in the amorphous structure The polymer and monomer mixture takes up the whole of the free pore volume Most of the residual monomer polymerises during the DSC scans above the glass transition temperature providing an excellent probe for the weak glass transition Polymerisation in the gel silica glass medium affects the glass transition temperature, the length of polymer chains, and the degree of polymerisation Keywords: crylate gel silica glass, thermal analysis, glass transition temperature, polymerisation, poly-methylmetha- Introduction Composites based on porous gel silica glasses impregnated with PMMA have been prepared by the formation of a porous sol-gel glass followed by in-situ polymerisation from the monomer [l-4] These composites are used as host media in optical applications such as lasers, optics, sensors and nonlinear optical studies where matching of refractive index and reduced scatter is achieved and also where optically active components can be incorporated [5, 61 The composites have improved optical quality and mechanical strength, which is valuable for these applications The in-situ polymerisation conditions leads to the presence of a small percentage of residual monomer, There is evidence that hydrogen bonding, between the pore surface silanol groups and the polymer side chains, affects the stretching vibrational frequency of the C=O ester carbonyl group [3] It is noted that the degree of insitu polymerisation of MMA monomer inside the silica glass pores, is further affected by the silica gel pore environment The presence of residual monomer will affect the optical, thermal and mechanical properties of the composite It is also of concern, that the composite’s structure should provide an inert host for the optically active dopants, and that heating of the samples will not alter their physical characteristics, for example by encouraging monomer to polymer conversion to take place, which changes the properties while in use The effect of residual monomer on the thermal properties of the composite glass is, therefore, of great interest and will be explored in this work by thermal analysis The hydrogen bonding hypothesis between the polymer and pore surface functional silanol groups will be further investigated by simple considerations regarding the molecular dynamics in the composite system on the basis of thermal analysis data Previous thermal analysis reports [4, 71 have shown that the molecular dynamics of the polymer and the silica glass, typically of lo-15 nm pore size, affect the glass transition temperature of the polymer As a result the glass transition temperature of the PMMA decreases 204 Pallikari-Viras, Li and King by about 20°C when it polymerises inside the silica glass pores Such a behaviour will affect the temperature range of applications of these composite materials and, similarly, their performance It is, therefore, of interest to examine the previous thermal analysis observations, in a range of composites of smaller pore size, in this case of the order of nm It will be shown that, unlike in previous observations, the glass transition temperature of the polymer in the glass pores was found to be higher than that of the bulk An explanation of this difference will be given For analysis purposes the benefits of the presence of a small percentage of residual monomer, as a probe of weak structural transformations, will be discussed source was a Coherent Innova 90 argon laser operated at 514.5 nm and 700 mW Typical operating conditions were bandwidth = cm-‘, scanning increment = cm-‘, integration time = s The frequency scale was calibrated by reference to the spectra of L-cystine and the 812 cm-’ line of PMMA Thermal analysis measurements of comparable mass samples (ranging from 2.28 to 4.74 mg) were performed with a DSC 220°C Seico Instruments Inc calorimeter The heating and cooling rates were maintained at 10 degrees/min Thermal analysis on a number of PMMA/gel silica glass composites and PMMA bulk samples show repeatable characteristics in the region from 25 to 250°C Figs to There are three temperature regions where exothermic peaks are observed: the first and stronger peak having a maximum at about 116°C in the PMMAonly samples and at about 120°C in the PMMA/gel silica glass composites, the second at about 160°C and the third at about 200°C Table These are attributed, as it will be shown later, to the polymer in the composite The most prominent feature of the DSC curves, the strong exothermic peak, is found at the lower temperature region near the glass transition temperature to amorphous PMMA (105C) The samples’ glass transition temperatures have been recorded in the second heating (and previous cooling) run, occurring at about 105°C for the PMMA-only, Figs 2(d), 3(c) and about 114°C in the composite samples, Figs 2(c), 3(d) It was observed that the onset of the first exothermic peak, Ti , Figs and coincides with the corresponding samples glass transition temperature In fact, the composite 3.1 Experimental Porous gel silica glass samples partially densified at temperatures of 600°C and 700°C and 800°C were supplied by Geltech Inc (USA) The procedure used for the in-situ polymerisation of MMA in silica glass is described elsewhere [3,8] The polymer samples were polymerised (a) inside the gel silica glass pores, for example, labelled PMMA/silica 600 for sol-gel glass samples densified to 600°C and similarly for the 700°C and 800°C densified samples, (b) polymerised external to the gel silica glass but inside the same polymerisation vessel, e.g., labelled PMMA only-600 for conditions corresponding to the 600°C glass sample The instrumental and measurement details for the gel silica glass pore characteristics obtained by nitrogen adsorption and for the polymer molecular weights obtained by GPC (see Table 2), are described in reference [3] Raman spectra were recorded by means of a computer operated Spex Ramalog spectrometer fitted with a 1403 double monochromator and a 1442U third monochromator in the scanning mode The light Table Results Thermal Analysis Measurements 1, Data evaluated from DSC and nitrogen adsorption measurements Tl T2 T3 (“C) (“C) -ho J/g -h, (“C) Sample fl fl fl f1.5 f1.5 PMMA only-600 PMMA only-700 PMMA only-800 PMMAkilica 600 PMMAkilica 700 PMMA/silica 800 104 105 103 118 106 113 142 176 160 159 164 160 191 200 200 200 200 200 22.0 28.0 20.0 J/g 11.7 14 10.2 b/H n= (%) k/h/> f0 03 10.20 Vd/(l + Vd) 39 49 3.6 053 0.52 0.51 0.514 0.484 0.481 Thermal Analysis I I I 50 100 150 I 200 205 i Heatingtemperature(“C) Figure I DSC first heating curves of PMMA/gel silica glass composite samples from substrates densified at: (a) 600°C mass of composite m = 3.14 mg, (b) 7OO”C, m = 4.15 mg, (c) 800°C m = 2.85 mg 50 100 150 200 Heatingtemperature(“C) Frgure DSC heating curves of: (a) PMMALsilica 600 m = 3.14 mg, first run, (b) PMMA only-600, mass of bulk, m = 2.28 mg, first run, (c) PMMA only-600, second run, (d) PMMA/silica 600 second run samples glass transition and Tt onset temperatures shift to higher values by the same amount The two other peaks found in the DSC curves, having maxima at T2 and T3, are generally much weaker in intensity and broader and appear at about the same temperature for both PMMA-only and PMMA/gel silica glass composite samples These temperatures are near the melting points of the crystalline isotactic (160°C) and syndiotactic (200°C) PMMA [9] The consecutive heating (and previous cooling) curves not display the above exothermic peaks No exothermic peaks were observed in the DSC scans of silica only samples in the temperature region of 25250°C TY a 3.2 , I 50 100 Molecular Weight Measurements ,\\ 150 200 Heatingtemperature(“C) Figure DSC heating curves of: (a) PMMA/silica 800, m = 2.85 mg, first run, (b) PMMA only-800, m = 4.74 mg, first run, (c) PMMA/silica 800, second run, (d) PMMA only-800, second run The samples’ molecular weights determined by GPC are shown in Table with nitrogen adsorption measurements of surface area and pore size The GPC measurements indicate that the polymer chains inside the glass pores grow smaller than the chains in the corresponding PMMA-only sample Although the pore 206 Pallikari-Viras, Li and King Table Data evaluated Surface area (m*/g) f20 Sample PMMA from nitrogen adsorption Pore volume (cm343 f0.02 and GPC measurements Mean pore diameter (nm) only-600 M, flO% Mw flO% 290,000 840,000 PMMAkilica 600 605 0.89 5.9 4,000 56,000 PMMAkilica 700 587 0.79 5.4 7,000 73,000 PMMA/silica 800 557 0.78 5.6 6,000 67,000 environment would create polymerisation conditions under excess pressure, a situation which will encourage the formation of higher molecular weights [lo], at the same time it will reduce the propagation mobility of the free radicals Since there is less available space inside the pores the free radical propagation is restricted and smaller polymer chains may be expected to be formed The molecular size will also depend on the degree of polymerisation, which is lower in the PMMA/gel silica glass composites, according to the Raman data The molecular size of the polymer inside the glass pores, compared with those in the PMMA-only samples, will depend on one or more of the above factors, i.e., the early termination of the polymerisation due to the effect of glass on mobility, the excess pressure effect and incomplete polymerisation due to the restricted space The resulting molecular size will depend upon which factors most dominate the polymerisation process It should be noted, however, that the reverse phenomenon (i.e., the polymer in PMMA/gel silica glass samples having higher molecular weight than in the respective PMMA-only samples) has been observed in previous work, for the case where the glass pore size was smaller (d = 4.4 nm) [3] and where there were probably excess pressure effects influencing predominantly the chain length relative intensity of the two peaks may be analyzed as an approximate measure of the percentage of monomer in the monomer/polymer mixture The Raman spectra show that not only is polymerisation incomplete in all samples, but also that the degree of conversion is lower in the PMMA/gel silica glass composites 3.3 Raman Spectroscopy Measurements A typical Raman spectrum of a PMMA-only 800 sample is shown in Fig over the region 1200-1800 cm-‘ The most interesting features in this frequency region for this study, are the two peaks at -1642 cm-’ and w 1725 cm-‘ The peak at 1642 cm-’ indicates the presence of monomer (MMA), H2C=C(CHs)COOCHs, which has not been converted to PMMA during polymerisation The peak at 1725 cm-’ is assigned to the C=O stretching mode of the ester carbonyl The B S ,*I 10 Raman shift (cm-‘) Figure Raman spectrum of sample the 1642 cm-’ and 1725 cm-’ lines PMMA only-800, showing Discussion The DSC exotherms are attributed to a polymerisation reaction There are several reasons that support this explanation Ran-ran spectroscopy and NMR data show that the PMMA is not fully polymerised in either the PMMA/gel silica glass composites or the bulk PMMA, prepared under the same temperature, pressure and initiator concentration conditions [3] The other possibility, that the peak may be due to crystallisation reaction, can be excluded on the basis that no Thermal Analysis melting peaks are observed around the expected melting points, releasing the same amount of energy as the exothermic peaks A third reason in favour of the polymerisation against crystallisation interpretation, is that the exothermic peak disappears upon consecutive heating (and cooling) of the samples, Figs and The glass transition is, however, present in the re-heating (and cooling) curves This behavior is to be expected if a near 100% conversion of monomer to polymer has occurred upon first heating, rather than crystallisation We assume, therefore, that further polymerisation occurs during the first DSC heating run, during which the monomer is more fully converted into polymer There is, nonetheless, the presence of the two weak exothermic peaks, which notably occur around the melting point temperatures of the iso- and syndioPMMA The above observations lead to the following interpretation The in-situ polymerisation temperature of 60°C of our samples is well below the glass transition temperature, which is 105°C for amorphous PMMA [9] As the polymer concentration increases during polymerisation the reaction medium becomes more viscous at T < T,, the polymerisation rate decreases and the conversion is forced to stop before it is fully completed (cf Trommsdorff or glass effect) [lo] As a result, there will be monomer units trapped within the polymer chains, the greatest proportion of them in amorphous polymer regions While the temperature rises near the glass transition, during the first heating run, the amorphous polymer chains become flexible, the monomer is freed and the prior incomplete polymerisation now continues until almost all monomer trapped in the amorphous region is fully converted The polymerisation would be expected to commence at the glass transition of the monomer/polymer mixture, which will be slightly lower (due to presence of unconverted monomer units acting as plasticiser [Ill) than the glass transition of the pure polymer This is confirmed, as the glass transition observed in there-heating (and cooling) curves of the samples coincides with the onset temperature of their first exothermic peak, Figs and The weak exotherms at about 160°C and 200°C are likely to have the same origin as that of the strong exothermic peak, i.e., they are polymerisation peaks, but come from fewer monomers trapped in areas of different chain structure It is obvious that, although the incomplete polymerisation is not desirable, as far as the reinforced silica glass optical quality is concerned, it serves, however, as a sensitive probe of the weak 207 glass transition More information can be drawn from the polymerisation peaks, about (a) the percentage of monomer to polymer in the in-situ polymerisation, (b) the extent by which the pores are filled with the polymer, and (c) providing some insight into the polymer chain dynamics within the gel silica glass pores 4.1 Estimation of the Extent of Polymerisation The degree of conversion of monomer to polymer in the samples can be estimated from the enthalpies of polymerisation The enthalpy of conversion of liquid monomer to amorphous or (slightly) crystalline polymer is H = -560 J/g of polymerised monomer (91 We will assume that a fully converted monomer under the conditions met in this work, has the same enthalpy of polymerisation H If m, is the mass of monomer which has not been converted and m,, is the mass of the polymer in the (pre-heated) PMMA-only samples then the ratio of monomer will be: mm/(mp + m,) If hb(Jg-‘) is the enthalpy of the bulk PMMA-first polymerization peak (most monomers are trapped in the amorphous region), then the ratio hh/H will yield the percentage of monomer in the amorphous part of the polymer, or, approximately, in all the polymer The estimated percentage of unconverted monomer in PMMA-only samples from the thermal analysis data, hb/H, is of the order of 4%, Table This value is of the same order as previous estimation from NMR data of similar samples 131 4.2 Estimation of the Polymer Content in Glass The enthalpies of polymerisation of the PMMA/gel silica glass composites combined with pore geometry characteristics can yield information about the extent of filling of the glass pores with polymer If V = volume of pores/mass of glass, M and m represent the mass of the glass and polymer is the composite, respectively, and II = m/(M + m), is the mass ratio of polymer content in the composite samples, then, V volume of polymer/mass of glass, since, volume of pores volume of polymer in glass If the density of the polymer is d = mass of polymer/volume of polymer, then: Vd Vdtl =-l-l - (1) 208 Pallikari-Viras, Li and King The value of II can be estimated from the enthalpies of polymerisation, h, and hb, of the composite and bulk PMMA samples, respectively as (2) Also correcting the value of h, for the mass of the polymer, yields the enthalpy of the PMMA-only polymerisation peak, h c M+m m =h h (3) Table shows that the data of the last two columns satisfy relation (1) where the polymer density is taken as d = 1.19 gcmp3 [9] According to (1) and the data of Table 1, the pore volume appears to be fully taken up by the polymer, (which is about 50% of the composite), since the values of the two last columns are in agreement The mass of the small percentage of liquid monomer present may give a noticeable polymerisation peak, but it is negligible as far as its volume in the pores is concerned 4.3 Chain Dynamics and Sutjace Phenomena It has been suggested in earlier work that H-bonding occurs between the polymer side chain and the silanol (SiOH) groups inside the glass pores [3] The Hbonding hypothesis was based on Raman spectroscopy data The DSC results of this work support the above hypothesis The shift of Tg of the polymer in composites to higher temperatures, taking into account the expected shift to lower temperatures due to the presence of the monomer, is the result of the hindering of the segmental motions of the macromolecular chain due to the H-bonding These increased excess surface forces, through the pressure that they exert, reduce the flexibility of the polymer chains and lead to higher glass transition values H-bonding effects are predominant in glasses of very small pore sizes In larger pore sizes other effects (e.g., the lower degree of cross linking) may cause an overall decrease of the transition temperature In gel silica glass samples having very similar surface areas, the average pore size may play an important part in the molecular interaction, such as the H-bonding between the silanol groups and polymer chains [12] and, therefore, affect the glass transition temperature of the polymer in the pores The densification temperatures of 600, 700 and 800°C of the measured silica glasses are relatively close and can result in similar pore sizes, as shown in Table Heating the composite above 200°C will result in a small degree of depolymerisation of PMMA The process is slow compared to the thermal treatment rates [ 131 The few monomers created will be in equilibrium with the rest of the polymer inside the sealed vessel The subsequent cooling of the sample to ambient temperature, will result in a near fully converted polymer The glass transition step is, therefore, observed in the second heating run It is important for a number of applications that the glass transition temperature of PMMA in composites is not affected by thermal treatments Higher glass transition temperatures will be favourable In that sense, the in-situ polymerised samples have slightly improved stability as well as optical properties, having increased their glass transition by approximately 10-15 degrees Conclusions Thermal analysis measurements support the hypothesis of hydrogen bonding between polymer side groups and surface silanol groups in PMMA/gel silica glass composites as suggested in earlier work The Hbonding causes the glass transition of PMMA to increase by 10-15 degrees and, therefore, improves its thermal stability against deformation while heating leading to useful improvement in its optical quality Allowance should be made for the small depression of the glass transition due to the presence of residual monomer The observed glass transition temperature of the bulk PMMA samples are characteristic of amorphous PMMA The polymer chains inside the silica glass were found to be smaller than the chains of the bulk PMMA prepared under the same conditions of polymerisation As the mobility of the polymer chains and of the initiator inside the glass pores decreases, due to reduced available space, smaller molecular chains are formed The whole volume of the pores is practically taken up by the polymer and contains about 4% of unconverted monomer, which further polymerises during the first heating DSC run The full polymerisation of the residual monomer takes place above three characteristic temperatures: the glass transition temperature of Thermal Analysis the amorphous polymer and the melting points of isoand syndiotactic PMMA The last two polymerisation peaks are due to monomer trapped within crystallites of iso- and syndiotactic chains Therefore, the unconverted monomer provides a useful, sensitive and simple probe, in a DSC scan, to pick up the weak glass transition and the presence of crystallinity in the amorphous polymer Acknowledgments This work has been financially supported by the European Science Exchange Program of the Royal Society and by the University of Manchester We express special thanks to Dr Colin Booth and Dr Peter M Budd of the Chemistry Department, University of Manchester, and Professor Alan H Windle of the Department of Material Science and Metallurgy, Cambridge University, for elucidating discussions on polymer properties View publication stats 209 References E J.A Pope and J.D Mackenzie, Mat Res Sot Bull 12, 29, (1987) E.J.A Pope, J Sol-Gel Sci &Tech 2, 717 (1994) X Li, T.A King, and E Pallikari-Viras, J Non-Cryst Solids 170,243 (1994) E.J.A Pope, M Asami, and J.D Mackenzie, J Mat Res 4, 1018 (1989) M Rahn and T.A King, Appl Opt (1996), 34, 8260 G J Gall, X Li, and T.A King, J Sol-Gel SCI & Tech 2, 775 (1994) B Abramoff and L.C Klem, m 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SCI & Tech 2, 775 (1994) B Abramoff and L.C Klem, m Chemical Pmcessq of Advanced Materials, edited by L.L Hench and J.K West (Wiley, New York, 1992) p 73 X Li and T.A King, J Sol-Gel Sci & Tech... the amorphous polymer Acknowledgments This work has been financially supported by the European Science Exchange Program of the Royal Society and by the University of Manchester We express special... Department, University of Manchester, and Professor Alan H Windle of the Department of Material Science and Metallurgy, Cambridge University, for elucidating discussions on polymer properties