Characterizing Mesh Size Distributions (MSDs) in Thermosetting

ARTICLE pubs.acs.org/JPCB

Characterizing Mesh Size Distributions (MSDs) in Thermosetting Materials Using a High-Pressure System J.-F. Larche,†,‡,§ J.-M. Seynaeve,†,‡,§ G. Voyard,†,‡ P.-O. Bussiere,*,‡,|| and J.-L. Gardette†,‡ †

)

Laboratoire de Photochimie Moleculaire et Macromoleculaire, Universite Blaise Pascal, Clermont Universite, BP 10448, F-63000 Clermont-Ferrand, France ‡ Laboratoire de Photochimie Moleculaire et Macromoleculaire, CNRS, UMR 6505, F-63173 Aubiere, France § Direction Technique et Industrielle, PSA-Peugeot-Citro€en, Route de Gisy, 78140 Velizy Villacoublay, France Laboratoire de Photochimie Moleculaire et Macromoleculaire, Ecole Nationale Superieure de Chimie, Clermont Universite, BP 10448, F-63000 Clermont-Ferrand, France ABSTRACT: The thermoporosimetry method was adapted to determine the mesh size distribution of an acrylate thermoset clearcoat. This goal was achieved by increasing the solvent rate transfer by increasing the pressure and temperature. A comparison of the results obtained using this approach with those obtained by DMA (dynamic mechanical analysis) underlined the accuracy of thermoporosimetry in characterizing the macromolecular architecture of thermosets. The thermoporosimetry method was also used to analyze the effects of photoaging on cross-linking, which result from the photodegradation of the acrylate thermoset. It was found that the formation of a three-dimensional network followed by densification generates a modification of the average mesh size that leads to a dramatic decrease of the meshes of the polymer.

1. INTRODUCTION Thermosetting materials, or thermosets, are an important class of polymers that are cured by the heat from a chemical reaction or exposure to irradiation. The resulting three-dimensional network leads to a dramatic improvement in the material properties, including mechanical, chemical, and barrier properties. This improvement in material properties depends largely on the extent of cross-linking. Therefore, a characterization of the cross-link density, which is always a challenge, is required. In thermosets, this quantity is typically determined by measuring the swelling rate of the network by gravimetric analysis, DMA (dynamic mechanical analysis), PAS (positron annihilation spectroscopy), or NMR (nuclear magnetic resonance) spectroscopy.17 Recently, a calorimetric method known as thermoporosimetry (or thermoporometry), which allows for textural analysis of materials, has been adapted to measure the mesh size distribution of polymers.810 Porous media such as silica and alumina were the first materials investigated using the thermoporosimetry technique.10,11 Their pore size distributions (PSDs)12 and porous volumes (Vp)13,14 were precisely calculated. The concept was then extended to the polymer field because, inside a swollen polymeric gel, the solvent behaves exactly like a liquid trapped in a porous material. Notably, the thermal transitions (liquid to solid and also solid to solid) are shifted in the same way as they would be in porous media. This method is now used for the network assessment of polymeric materials, in particular, for the case of elastomeric compounds.1517 r 2011 American Chemical Society

A new application for this method, in determining the crosslinking reactions that occur when polymeric materials are subjected to photoaging, has also been reported.18 In a recent study concerning the aging of poly(vinyl carbazole) (PVK) materials,19 1,4-dioxane, which is a good solvent for the un-cross-linked polymer, was chosen as the swelling agent for the cross-linked material. Thermoporosimetry is based on the measurement of the shift in temperature (ΔT) of the thermal transition undergone by a liquid when confined inside a divided medium. In the case of polymers, one measures the temperature change of the phase transitions of the solvent when it is confined inside the network compared to that of the free solvent outside and at the surface of the network. To carry out such measurements, the solvent must penetrate the polymer, which can be difficult in the case of thermosets. The problem stems from the degree of swelling that can be reached or from the relatively low solvent diffusion rate.20 The consequence is a poor rate of the confined solvent as compared to that of the free solvent, which can dramatically limit the sensitivity of the method, making this informative method difficult to apply in the case of thermosets. To overcome this obstacle, the rate of solvent transfer into the thermoset must be increased. Two different methods can be used and are Received: July 19, 2010 Revised: March 7, 2011 Published: March 23, 2011 4273

dx.doi.org/10.1021/jp106699u | J. Phys. Chem. B 2011, 115, 4273–4278

The Journal of Physical Chemistry B

ARTICLE

Table 1. Five Clearcoat Samples Used in This Study

a

Figure 1. Representation of the idealized structure of the cross-linked acrylicmelamineurethane network.

reported in this article: the first is based on a microwave heating process, and the second is based on pressurization of the solvent. The application of both of these techniques to the characterization of cross-linked polyacrylates, which are thermosetting materials used as automotive clearcoats, is described. In addition, the article reports the effect of exposure to UV light on the characteristics of the network.

2. EXPERIMENTAL SECTION 2.1. Polymer. The samples correspond to a typical clearcoat used for automotive applications. The different samples were based on a classical acrylate-based polymer cross-linked with two types of cross-linkers (melamines and isocyanates). Figure 1 illustrates the idealized structure of the crosslinked network, and Table 1 lists the five samples used in this work. The five samples (samples 14 and A) studied were provided by three suppliers and donated by PSA Peugeot Citro€en. The four reference samples 14 were used to verify the reproducibility of the measurements of the apparatus because of their very close chemical structures and architectures. Most of the study was carried out with sample A, especially for the comparison between DMA and thermoporosimetry and for the understanding of the impact of photoaging on the MSD (mesh size distribution). A mixture containing the required amounts of the liquid matrix and hardeners was bar coated on Tedlar sheets and then cured in a ventilated oven at 140 °C for 20 min to achieve the cross-linking reaction. Cured films with a thickness between 40 and 60 μm were obtained and separated from the Tedlar sheet, to obtain samples in the form of a freestanding film. Poly(dimethylsiloxane) (PDMS) was provided by Scientific Polymer Products (Ontario, NY). 2.2. Methods. Exposure to UV light was carried out in an artificial aging device, SEPAP 12/24, from Atlas.21 This unit was equipped with four medium-pressure mercury lamps (Novalamp RVC 400W) situated in a vertical position at each corner of the chamber. Wavelengths below 295 nm were filtered by the glass envelope of the sources. The temperature at the surface of the samples was fixed at 60 °C. Dynamic mechanical analysis measurements were performed on a DMA Q800 from TA Instruments. The polymeric film was tested in tensile mode (temperature range, 100 to 150 °C; heating rate, 3 °C/min; thickness of the film, 40 μm; strain frequency, 1 Hz; deformation, 0.2%).

sample

resins

cross-linkersa

supplier

1

polyacrylate, styrene

urethane (M), melamine (m)

W-PSA

2



X- PSA

3



Y-PSA

4



Z-PSA

A


melamine (M), urethane (m)

PSA

M, majority component; m, minority component.

The measurement of the ΔT shift between the free and trapped solvent was carried out with a Mettler Toledo DSC 822 apparatus equipped with a liquid nitrogen cooling system, with cooling from 25 to 100 °C at a rate of 0.5 °C/min. Analyses were performed on swelled films with an initial thickness of 40 μm, which were placed in a 160 μL aluminum crucible. According to the GibbsThomson relationship, when a liquid is trapped inside a cavity with a characteristic size of Rp, its crystallization temperature undergoes a depression Tp T0 as follows ΔT ¼ Tp T0 ¼

2σSL ðcos θÞT0 k ΔHm Rp ΔHm Fs Rp

ð1Þ

where Tp is the crystallization temperature of a confined liquid, T0 is the normal melting temperature of the liquid, σSL is the surface energy of the solid/liquid interface, θ is the contact angle, ΔHm is the melting enthalpy, Fs is the density of the solid, and k is a constant. The solvent volume (dVp/dRp) as a function of the pore radius (Rp) was obtained with eqs 4 and 5, as described elsewhere. The equations derived from Brun et al.10 are Rp ¼ A

B ΔT

dVp cΔT 2 Y ðTÞ ¼k Wa Rp dRp

ð2Þ ð3Þ

where k and c are calibration constants that depend on the instrument sensitivity, sample mass and heating rate. The apparent heat of fusion, Wa, is temperature-dependent. The main parameters of the equations were obtained from the calibration step, which allowed for calculation of the empirical parameters of 1,4dioxane, performed following the standard procedure reported in a previous work.19,22 From this calibration, one can write Rp ðnmÞ ¼

116:3 þ 0:46 ΔT

Wa ðJ=cm3 Þ ¼ 118:7 expð0:0186ΔTÞ

ð4Þ ð5Þ

The application of the thermoporosimetry method makes the assumption that the melting point of a solvent in a polymer network can be described by the GibbsThomson (GT) relationship and that the thermal trace in a differential scanning calorimeter (DSC) can consequently be analyzed using the appropriate relationship between the crystal size and the melting point and heat of fusion. This method is a powerful means of observing crystal size evolution, but it presents two limitations that suggest that thermoporosimetry needs to be used with caution, especially to make absolute crystal size determinations. The first is that, as reported in previous works, the GT relationship between the melting and size is not strictly valid, 4274

dx.doi.org/10.1021/jp106699u |J. Phys. Chem. B 2011, 115, 4273–4278


ARTICLE

Figure 2. Schematic representation of the home-built pressurization apparatus.

Figure 4. DSC thermal curve of swelling solvent in four samples of automotive clearcoat.

Figure 3. Mesh sizes for PDMS swelled at (A) ambient pressure and temperature and (B) 300 bar for 72 h.

not only for polymer networks but also for porous media in general. It is often found,23,24 in both networks and pores, that the value of the heat of fusion of the crystals decreases with decreasing pore size, which can be problematic because the crystal size is deduced from an equation that assumes a constant heat of fusion. The second limitation, as described in previous reports,23,25 is related to the fact that an organic solvent in a polymer should have a depressed melting point according to the FloryHuggins theory. Therefore, the GibbsThomson equation needs at least one term to include the depression of the melting point due to mixing thermodynamics. One approach26 shows the resulting potential difficulties and implies that the observed melting point depression could be interpreted to be greater than what one expects without necessarily invoking size effects. 2.3. Apparatus of Accelerated Diffusion. Two different techniques were used to increase the rate of transfer of the solvent into the polymer. In one approach, the increase of the solvent transfer rate necessary to obtain polymer swelling was obtained with a microwave heating process that was performed using a MARS5X (CEM Co.) microwave system. A polymer sample placed in the chamber was soaked for 10 h at 80 °C under microwave irradiation. The microwave power varied automatically from 0% to 100% (1600 W at 100%) according to the set temperature and pressure.27 Polymer swelling was also carried out by pressurization28,29 using the home-built apparatus depicted in Figure 2. This device was designed on the basis of high-performance liquid chromatography (HPLC) pump L-6200 from Merck. Valves, screws, ferrules, and tubing were Rheodyne HPLC materials. The pressurization cell (C) was an empty column (volume = 1.8 cm3). The pressure was measured by the system pressure transducer (P) from the HPLC pump. The solvent was channelled from the pump to the cell by a six-position switch valve. As

a function of the position of the valve, the cell was isolated from the HPLC pump, and the cell pressure was directly measured with the pressure transducer. The polymer sample to be analyzed was introduced into the empty cell. The cell was then hermetically sealed and connected to the switch valve and the pressure transducer. The valve was then turned to position 2. The solvent [1,4-dioxane (Aldrich Co.), HPLC quality and used without further purification] was then sent to the cell at a flow rate of 0.1 mL/min. To increase the pressure in the cell from 0 to 300 bar, outlet 4 was closed. When the working pressure was reached, the valve was switched to position 1. In this position, the cell and the pressure transducer were in a closed loop with a stable pressure. The sample was maintained in this configuration for almost 72 h.

3. RESULTS AND DISCUSSION 3.1. Validation of the Pressurization Apparatus. First, the efficiency of the method used to increase the rate transfer of a solvent into a polymer was checked on two cross-linked PDMS samples. One sample was swelled under atmospheric pressure, and the other was swelled under pressure (72 h at 300 bar), following the procedure described in the preceding section. In the case of PDMS, previous results have shown that the swelling equilibrium is reached only after several days.30,31 Using the calibration formerly reported,19 the mesh size distributions (MSDs) of the samples were calculated and compared to the results obtained with the method reported here. A comparison of the MSDs characterized by the two different procedures is presented in Figure 3. This figure shows that the MSDs of the two samples are similar (around 4.5 nm), which indicates that swelling under pressure does not modify the mesh size values. This first set of results indicates that the device and the method are valid for characterizing thermosetting materials. 3.2. Evaluation of the MSD of Automotive Clearcoat. As mentioned in the Introduction, thermosets are polymers with a high cross-linking rate. To facilitate the penetration of the solvent into the mesh of the polymer sample, the samples were swelled under pressure following the method working under pressure. The thermograms of the solvent introduced in the four acrylate clearcoats, samples 14, are shown in Figure 4. The thermograms show three distinct peaks labeled S, W, and C. S is the freezing peak of the free dioxane, and W corresponds to the freezing peak of the water traces. C is the peak of the confined solvent.19 4275



ARTICLE

Figure 5. MSD distribution of an acrylatemelamineurethane network (sample 2).

Figure 6. Storage modulus, E0 , of unaged clearcoat (sample A) obtained by DMA.

Table 3. E0 r and Mc Values for Sample A Table 2. Average Mesh Size of Automotive Clearcoat Determined by Thermoporosimetry As a Function of the Swelling Method (Pressure versus Microwave) sample

swelling method

E0 r (MPa)

Mc (g mol1)

41.3

240 ( 12

average mesh size (nm)

1

pressure

2.99 ( 0.1

2

pressure

2.90 ( 0.1

3

pressure

2.98 ( 0.1

4

pressure

2.78 ( 0.1

1

microwave

3.03 ( 0.1

4

microwave

2.91 ( 0.1

The insert in Figure 4 presents the C peaks that correspond to the four different samples that were studied. The differences between the freezing temperatures of each sample are relatively weak, which is a good indication of the reproducibility of the measurement and the accuracy of the method employed. These results clearly indicate that through this method, it is possible to characterize thermosetting materials. The distribution of mesh sizes of the samples was calculated, and a sample average mesh size was estimated. Figure 5 shows the results obtained in the case of sample 2. The results for the four samples are reported in Table 2. This table shows that the average mesh size is close to 2.9 nm and that the distribution is narrow. The fwhm (full width at half-maximum) is narrow for the four samples. Such a result is expected, given the structure of the three-dimensional network of thermosetting materials, because the cross-linking points are controlled by the chemical structure of the polymer and by the curing step. A second type of analysis was carried out using the other method based on the microwave-assisted transfer of the solvent. The polymer samples were immersed in 1,4-dioxane, placed in the microwave system, and then analyzed by DSC. The results are reported in Table 2. Notably, the average pore sizes obtained by the two methods are similar, which permits the validation of both methods used to swell thermosets and the analysis of the three-dimensional network by thermoporosimetry. 3.3. Comparison between DMA and Thermoporosimetry. A complementary technique was used to validate the results obtained by thermoporosimetry. This technique was based on the DMA (dynamic mechanical analysis) results. Figure 6 shows the storage modulus E0 as a function of temperature in degrees Celsius obtained by DMA for sample A.

Figure 7. Average Rp obtained by thermoporosimetry for sample A. Sample A was analyzed three times (traces R, β, and γ).

From the curve E0 = f(T/°C), the rubbery modulus E0 r (E0 on the rubbery plateaus) was obtained, which is correlated with the average molecular weight between two cross-link points (Mc) by the relationship32,33 Mc ¼

3FRT E0 r

ð6Þ

This expression is widely used to represent the modulus of a highly elastic polymer, but it requires certain assumptions, such as a random distribution of cross-links and the influence of free ends of molecules. It has been demonstrated34 that this model is not totally realistic because a proportion of the network does not participate in the elastic deformation. Table 3 gives the value of E0 r and the calculated average value of Mc for sample A. With the approximation of a chain that is composed of only CC bonds (CC length = 1.54 Å), an average length of approximately 2.6 nm between two cross-link points was obtained. This value was compared with the MSD value obtained from the thermoporosimetry measurements. The values of Rp are reported in Figure 7. Three analyses of the same sample are presented to ensure reproducibility of the measurement. Using this technique, Rp = 3.1 ( 0.1 nm was obtained, which is the same order of magnitude 4276



ARTICLE

Figure 8. (Left) Evolution of the average mesh size with exposure time (SEPAP 12/24, 60 °C). (Right) Thermogram and MSD of sample A exposed for 800 h (SEPAP 12/24, 60 °C).

Table 4. Evolutions of the Average Molecular Weight, Carbon Number, and Length between Two Crosslink Points with Exposure Time (SEPAP 12/24, 60°C) exposure

Mc

C number between

length between two

time (h)

(g mol1)

two cross-link points

cross-link points (nm)

0

240 ( 12

17

2.6

500

226 ( 12

16

2.4

750

197 ( 12

14

2.1

as the value obtained using DMA (3.1 versus 2.6 nm). The relatively weak difference between the two values can be explained by the effect of enlargement of the mesh due to the swelling effect of 1,4-dioxane.35 3.4. Impact of Photoaging on the MSD. The aim of this research was not only to characterize the mesh size distributions of a thermosetting material but also to follow their modifications and see trends when the polymer matrix was subjected to photoaging. Exposure to UV light in the presence of oxygen generates modifications of the chemical structure, which induces a loss of the polymer engineering properties. The modification of the chemical structure is accompanied by two major modifications of the macromolecular chains: chain scissions (with rupture of the macromolecular chain) and cross-linking (with formation of a new covalent bond between the macromolecular chains). These modifications are not easy to characterize, and thermoporosimetry was expected to provide valuable information. Samples of material A were exposed to artificial photoaging conditions for different durations ranging from 0 to 1000 h. Each sample was then swollen under pressure at 300 bar in 1,4-dioxane for 72 h following the procedure described above and then analyzed by the classical procedure. The MSD following the calculation procedure described in the previous section and the thermogram of the sample A, aged for 800 h, are shown in the Figure 8 (right). The evolution of the MSD with exposure time is presented in Figure 8 (left). For clarity, only five MSDs are shown. The results obtained show that the average size of the meshes resulting from UV-light irradiation in the presence of oxygen decreases with exposure time. The width of the distributions (fwhm) increases with aging time. These results indicate that photo-oxidation provokes cross-linking reactions, which, in turn, result in densification of the three-dimensional network.

Figure 9. Rp obtained by thermoporosimetry and length between two cross-link points obtained by DMA with exposure time.

Mc values were calculated using the results of DMA following the procedure described above. The results, including the average length between two cross-link points, are reported in Table 4. Mc decreases with irradiation time, which shows that DMA measurements show trends similar to the MSD evolution. It is obvious that cross-linking reactions take place in the clearcoat bulk during UV-light irradiation. The evolutions of the average length between two cross-link points were compared with thermoporosimetry measurements, as shown in Figure 9. The results in this figure show that the evolution of the MSD under aging measured by thermoporosimetry follows the decrease of the average length between two cross-link points measured by DMA, which confirms the good agreement between both these techniques to follow macromolecular architecture evolutions for sample A under aging.

4. CONCLUSIONS The results obtained in this work provide evidence that thermoporosimetry can be successfully applied to characterize mesh size distributions in the case of thermosets. Moreover, this method offers a great advantage in studying the effects of photoaging. We are in the process of extending thermoporosimetry studies (using the pressurization device) to a number of thermoset systems, to understand the impact of the curing process on their macromolecular architecture and, thus, on the desired mechanical properties. 4277



’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ33 473405518.

’ ACKNOWLEDGMENT J.-F.L. is supported by a Ph.D. fellowship from PSA-PeugeotCitro€en. The authors particularly thank D. Gavrel (PSA, Poissy, France) for her technical support and useful comments and advice and S. Farges (PSA, Sochaux, France) for her assistance with DMA measurements.

ARTICLE

(30) Virlogeux, F.; Bianchini, D.; Delor-Jestin, F.; Baba, M.; Lacoste, J. Polym. Int. 2004, 53, 163–168. (31) Delor-Jestin, F.; Tomer, N. S.; Singh, R. P.; Lacoste, J. Sci. Technol. Adv. Mater. 2008, 9, 024406. (32) Amdouni, N.; Sautereau, H.; Gerard, J.-F.; Pascault, J.-P. Polymer 1990, 31, 1245–1253. (33) Grillet, A. C.; Galy, J.; Gerard, J.-F.; Pascault, J.-P. Polymer 1991, 32, 1885–1891. (34) Charlesby, A. Int. J. Radiat. Appl. Instrum. C: Radiat. Phys. Chem. 1992, 40, 117–120. (35) Nedelec, J.-M.; Grolier, J. P. E.; Baba, M. Phys. Chem. Chem. Phys. 2008, 10, 5099–5104.

’ REFERENCES (1) Musto, P.; Ragosta, G.; Abbate, M.; Scarinzi, G. Macromolecules 2008, 41, 5729–5743. (2) Jean, Y. C.; Deng, Q.; Nguyen, T. T. Macromolecules 2002, 28, 8840–8844. (3) Zeng, M.; Sun, X.; Wang, Y.; Zhang, M.; Shen, Y.; Wang, B.; Qi, C. Polym. Adv. Technol. 2008, 19, 1664–1671. (4) Sagidullin, A. I.; Furo, I. Langmuir 2008, 24, 4470–4472. (5) Flory, P. J.; Rehner, J. J. J. Chem. Phys. 1943, 11, 521–526. (6) Boyard, N.; Vayer, M.; Sinturel, C.; Erre, R.; Levitz, P. Polymer 2005, 46, 661–669. (7) Shenoy, S.; Woerdeman, D.; Sebra, R.; Garach-Domech, A.; J. Wynne, K. Macromol. Rapid Commun. 2002, 23, 1130–1133. (8) Baba, M.; Gardette, J.-L.; Lacoste, J. Polym. Degrad. Stab. 1999, 63, 121–126. (9) Landry, M. R. Thermochim. Acta 2005, 433, 27–50. (10) Brun, M.; Lallemand, A.; Quinson, J.-F.; Eyraud, C. Thermochim. Acta 1977, 21, 59–88. (11) Billamboz, N.; Baba, M.; Grivet, M.; Nedelec, J.-M. J. Phys. Chem. B 2004, 108, 12032–12037. (12) Ishikiriyama, K.; Sakamoto, A.; Todoki, M.; Tayama, T.; Tanaka, K.; Kobayashi, T. Thermochim. Acta 1995, 267, 169–180. (13) Baba, M.; Nedelec, J.-M.; Lacoste, J. J. Phys. Chem. B 2003, 107, 12884–12890. (14) Maldonado, D.; Tanchoux, N.; Trens, P.; Galarneau, A.; Garrone, E.; Di Renzo, F.; Fajula, F. J. Porous Mater. 2007, 14, 279–284. (15) Iza, M.; Woerly, S.; Danumah, C.; Kaliaguine, S.; Bousmina, M. Polymer 2000, 41, 5885–5893. (16) Hay, J. N.; Laity, P. R. Polymer 2000, 41, 6171–6180. (17) Planes, E.; Chazeau, L.; Vigier, G.; Fournier, J. Polymer 2009, 50, 4028–4038. (18) Baba, M.; Nedelec, J. M.; Lacoste, J.; Gardette, J. L.; Morel, M. Polym. Degrad. Stab. 2003, 80, 305–313. (19) Bussiere, P. O.; Mailhot, B.; Rivaton, A.; Barthe, M. F.; Gardette, J. L.; Baba, M. Polym. Degrad. Stab. 2008, 93, 1376–1382. (20) Mensitieri, G.; Lavorgna, M.; Musto, P.; Ragosta, G. Polymer 2006, 47, 8326–8336. (21) Lemaire, J.; Arnaud, R.; Gardette, J. L. Rev. Gen. Caoutchouc Plast. 1981, 613, 87. (22) Bahloul, N. m.; Baba, M.; Nedelec, J.-M. J. Phys. Chem. B 2005, 109, 16227–16229. (23) Qin, Q.; McKenna, G. B. J. Polym. Sci. B: Polym. Phys. 2006, 44, 3475–3486. (24) Sun, J.; Simon, S. L. Thermochim. Acta 2007, 463, 32–40. (25) Wu, J.; McKenna, G. B. J. Polym. Sci. B: Polym. Phys. 2008, 46, 2779–2791. (26) Hoei, Y.; Ikeda, Y.; Sasaki, M. J. Phys. Chem. B 2003, 107, 1483–1490. (27) Inada, M.; Nishinosono, A.; Kamada, K.; Enomoto, N.; Hojo, J. J. Mater. Sci. 2008, 43, 2362–2366. (28) Bell, C. L.; Peppas, N. A. Int. J. Pharm. 1996, 134, 167–172. (29) Nahin, P. G. Clays Clay Miner. 1954, 3, 174–185. 4278


Characterizing Mesh Size Distributions (MSDs) in Thermosetting

Recommend Documents