We present the effect of organic modifier on crystallinity and nanomechanical properties of polymer clay nanocomposites (PCNs) using two different polymers while maintaining the same nanoclay and organic modifier. Experimental results and interaction energy maps of Polybutylene-Terephthalate- (PBT-) PCN system indicate that the underlying mechanisms of change in crystallinity and improvement in mechanical properties as proposed in altered phase theory are valid. Experimental and molecular simulation studies of PBT-PCN and Nylon6-PCN reveal that a higher crystallinity polymer could require significantly higher attractive and repulsive interaction energies between polymer and organic modifiers to change the crystallinity of the polymer in the PCN significantly and thus improve mechanical properties of the PCN.
Polymer clay nanocomposites (PCNs) are widely researched material systems for wide variety of applications in industry because of enhanced mechanical, thermal, optical, and barrier properties. PCNs are categorized as intercalated or exfoliated and are typically synthesized by adding small amount (about 1–9% of weight polymer) [
PBT is a semicrystalline thermoplastic polymer which shows excellent properties such as rate of crystallization, thermal stability and processability [
Na-montmorillonite with cationic exchange capacity of 76.4 mequiv/100 g was obtained from the Clay Minerals Repository at the University of Missouri, Columbia, MO, USA. 12-Aminolauric acid [NH2(CH2)11COOH] was supplied by the TCI America. Poly(1, 4-butylene terephthalate) (
Ten grams of Na-montmorillonite (Na-MMT) are crushed into fine powder and screened through #325 sieve (45
Five grams of PBT are added to 42 ml chloroform and 7 mL trifluoroacetic acid (TFA) and stirred for 2 hours to make a uniform solution. Subsequently, 0.45 g (9 wt% of OMMT with respect to pure PBT) OMMT is added to the PBT-CHCl3-TFA solution and then stirred vigorously for 3 hours to obtain PBT-PCN. PBT and PBT-PCN films are obtained by casting the corresponding solutions on a glass slide using a drawdown bar (Paul N. Gardner Company, Inc.) set at 25 mils thickness.
Ten grams of Nylon6 are added to 155.73 ml formic acid (FA) and stirred vigorously for 40 minutes to prepare a uniform solution. Next, 0.9 g (9 wt% of OMMT with respect to pure nylon6) of OMMT is dispersed into the nylon6-FA solution and stirred vigorously for 6 hours at room temperature. DI water is added to the resulting solution to form PCN and to remove the FA from PCN. Finally, the remaining DI water is removed by adding methanol anhydrous and the resulting solution is placed in a vacuum oven set at 313 K temperature for a period of 24 hours to obtain the PCN.
The X-ray analysis is performed by X-ray diffractometer (Philips Analatical X’pert MPD, Almelo, Netherlands) with secondary monochromator and Cu-tube by CuK
PA-FTIR experiments are conducted in FTIR bench (model Nexus, Thermo Nicolet, USA) with a photoacoustic cell (MTEC Photoacoustics model no. 300). Sample is prepared by pressing the OMMT powder into a pellet. The nylon6 sample is heated gradually from 303 K to 493 K and then pressed into a pellet. The PBT and PBT-PCN samples are prepared in smaller size and placed in the sample holder. The PA-FTIR is conducted in the linear photoacoustic mode for 500 scans at a resolution of
DSC experiments are conducted using differential scanning calorimeter (model Q1000 DSC) of TA instruments, New Castle, Delaware, USA. PBT and PBT-PCN samples are heated from 25°C to 250°C with nitrogen atmosphere with the heating rate of 10°C/min for heating/cooling/heating cycle with retention time of 3 minutes at the end of each treatment for temperature equilibration. Nylon6 and Nylon6-PCN samples are heated from 30°C to 100°C in 3 minutes and the temperature is maintained at 100°C for 10 minutes. A heating rate of 10°C/min is used for the first heating and cooling cycles with retention time of 10 minutes at 250°C and the heating rate of 20°C/min [
Nanoindentation experiments are conducted using triboscope nanoindenter (Hysitron Inc., Minneapolis, MN). The triboscope is operated in association with a multimode AFM controlled by the nanoscope IIIa controller and with the J-type piezo scanner from Veeco (Veeco Metrology Group, Santa Barbara, CA). For PBT and PBT-PCN, load controlled nanoindentation tests are performed using a 100–200 nm Berkovich (three-sided pyramid) diamond indenter tip. Experiments are conducted for maximum load of 2900
Molecular dynamics (MD) is a computational method used to study the position and interaction of atoms and molecules. MD is used extensively to study proteins, bio molecules, polymers, and minerals. MD provides a good understanding of molecules with respect to their interactions and molecular structure. This method involves solving the equation of motion in small time steps. The equation of motion is
Force field describes the potential energy of a molecular system. Potential energy of any molecular system consists of bonded energy and nonbonded energy. Bonded energy consists of bond, angle, dihedral, and improper energy of the molecular system. Nonbonded energy consists of van der Waals and electro static energy of the molecular system. Total potential energy of the molecular system can be described by the following expression
Materials Studio software, developed by Accelrys Software Inc., is used to build the molecular models. This software is widely used to construct molecular models. NAMD2.5b2 [
OMMT model consists of two sheets of montmorillonite (MMT) and eighteen numbers of 12-aminolauric acid molecules. Size of the OMMT model is 31.68 Å in
Initial OMMT model.
Distribution of partial charges (a) 12-aminolauric acid, (b) PBT, (c) Nylon6.
PBT monomers are used to build polymers. A representative molecular model of PBT is shown in Figure
Single chain molecular model of PBT.
Initial OMMT model is minimized for 25 ps at a temperature of zero Kelvin in vacuum using molecular dynamics program NAMD. Next, the temperature of the OMMT system is raised to 300 K in three equal steps by Langevin temperature parameter. Once the temperature is increased to 300 K, pressure of the OMMT system is increased to 1.01325 bars in four equal steps by Langevin piston pressure control. Next, the system temperature is increased by 33 K and subsequently reduced to atmospheric temperature. The increase and reduction of temperature is done to mimic the experimental conditions as mentioned in Section
Selected OMMT model, as shown in Figure
Estimated mass percentage of PBT with respect to clay.
Model | Intercalated polymer wt% with clay |
---|---|
PCN(1) | 1.96 |
PCN(2) | 4.19 |
PCN(3) | 6.41 |
PCN(4) | 8.64 |
PCN(5) | 10.87 |
PCN(6) | 13.10 |
Final OMMT model.
Initial PBT-PCN model.
Final PBT-PCN model.
Initial Nylon6-PCN model.
Final Nylon6-PCN model.
Molecular simulation of PCN is conducted for PBT-PCN and Nylon6-PCN system. Procedure for minimization and dynamics of PCNs is done as described in Section
Experimental
Sample |
|
| |
Pure MMT | 11.56–12.25 |
OMMT | 15.60–17.43 |
PBT-PCN | 13.90–14.53 |
Nylon6-PCN | 13.50 |
In our previous work, we have observed that the organic modifier, 12-aminolauric acid, is intercalated between clay sheets with hydrogen bonds and ionic interactions generated by end functional groups of organic modifier with surface oxygen of interlayer clay sheets. Nanocomposites are prepared by intercalation of polymer and MMT with the aid of organic modifier [
Band assignments of photoacoustic FTIR spectra of OMMT, PBT, and PCN.
Sample | Wave number range (cm-1) | Bands | Reference |
| |||
3646–3628 | O–H stretching of structural hydroxyl group | [ | |
3256–3035 | Ionic bonded N–H stretching | ||
2932 | C–H asymmetric stretching | ||
2853 | C–H symmetric stretching | ||
1713–1701 | C=O stretching | ||
1623 | Combination of O–H deformation and N–H bending | ||
1511 | Asymmetric R–COO- stretching | ||
1466 | CO–H bending | ||
1396 | Symmetric R–COO- stretching | ||
1112 | Si–O in-plane stretching | ||
1060 | Si–O out-of-plane stretching | ||
OMMT | 1022,997 | Si–O in-plane stretching | |
963 | Si–OH vibrations | ||
921 | Al–O/Al–OH stretching vibration | ||
884 | Al–FeOH deformation | ||
847 | Al–MgOH deformation | ||
799 | Si–O stretching of quartz and silica | ||
727 | Si–O deformation perpendicular to optical axis | ||
699 | Si–O deformation parallel to optical axis | ||
623 | Coupled Al–O and Si–O out-of-plane vibration | ||
531 | Al–OSi deformation | ||
478 | Si–OSi vibration | ||
| |||
3417 | –OH stretching | ||
3052 | Aromatic C–H stretching | [ | |
2962 | C–H2 stretching | [ | |
2895 | Aliphatic C–H asymmetric stretching | [ | |
2863 | Aliphatic C–H symmetric stretching | [ | |
1722 | C=O stretching | [ | |
1583 | Aromatic C=C stretching | [ | |
1504 | Aliphatic C–H2 bending | [ | |
1453 | Aliphatic C–H2 bending | [ | |
1407 | Aliphatic aromatic C–H in-of-plane bending | [ | |
PBT | 1387 | CH2 wagging | [ |
1281 | (C=O–)–O stretching | [ | |
1110 | CH2–O stretching | [ | |
1017 | Aromatic C–H bending | [ | |
873 | Aromatic C–H bending | [ | |
808 | Aromatic C–H bending | [ | |
729 | Aromatic C–H out-of-plane bending | [ | |
685 | Aromatic =C–H out-of-plane deformation | [ | |
633 | Aromatic in-plane deformation | [ | |
497 | Aromatic out-of-plane deformation | [ | |
476 | Aromatic out-of-plane deformation | [ | |
| |||
3633–3679 | O–H stretching of structural hydroxyl group-OMMT | [ | |
3413 | –OH stretching | ||
3260–3047 | Coupled N–H stretching and aromatic C–H stretching | [ | |
2963 | C–H2 stretching | [ | |
2895 | Aliphatic C–H asymmetric stretching | [ | |
2858 | Aliphatic C–H symmetric stretching | [ | |
1719 | C=O stretching | [ | |
1576 | Aromatic C=C stretching | [ | |
1503 | Aliphatic C–H2 bending | [ | |
1452 | Aliphatic C–H2 bending | [ | |
1406 | Aromatic C–H in-of-plane bending | [ | |
PCN | 1387 | CH2 wagging | [ |
1278 | (C=O–)–O stretching | [ | |
1105 | CH2–O stretching | [ | |
1020 | Aromatic C–H bending | [ | |
873 | Aromatic C–H bending | [ | |
810 | Aromatic C–H bending | [ | |
729 | Aromatic C–H out-of-plane bending | [ | |
685 | Aromatic =C–H out-of-plane deformation | [ | |
652 | Aromatic in-plane deformation | [ | |
624 | Tripled Al–O, Si–O out-of-plane and aromatic in-plane deformation | [ | |
497 | Aromatic out-of-plane deformation | [ | |
463 | Si–OSi deformation | [ |
Photo acoustic FTIR spectra of OMMT, PBT, and PCN in the energy range of 4000–400
Photo acoustic FTIR spectra of OMMT, PBT, and PCN in the energy range of 3800–2800
Photo acoustic FTIR spectra of OMMT, PBT, and PCN in the energy range of 2000–1000
Photo acoustic FTIR spectra of OMMT, PBT, and PCN in the energy range of 1000–400
DSC tests are conducted on the pure PBT and PCN and results are shown in Table
DSC results of PBT, Nylon6, and PCNs.
Materials | Enthalpy (J/g) | Crystallinity (%) |
---|---|---|
PBT | 50.21 | 35.6 |
PBT-PCN | 49.56 | 35.1 |
Nylon6 | 56.06 | 27.61 |
Nylon6-PCN | 46.81 | 23.05 |
In the present work, nanoindentation tests for PBT and PBT-PCN are performed in load control mode. Figure
Nanomechanical properties of PCNs.
Parameters | PBT | PBT-PCN | Nylon6 | Nylon6-PCN |
---|---|---|---|---|
Crystallinity (%) | 35.6 | 35.1 | 27.6 | 23.1 |
Elastic modulus (GPa) | 1.40 | 2.20 | 3.35 | 5.46 |
Hardness (GPa) | 0.09 | 0.14 | 0.12 | 0.17 |
Representative load displacement (L-D) curves for PBT and PCN.
Interaction energies between different constituents of OMMT and PCNs are calculated using NAMDEnergy module of NAMD software. Nonbonded interactions, van der waals and electrostatic, are calculated in terms of attractive and repulsive interactions. Among different initial
Nonbonded interaction energies of OMMT constituents.
OMMT constituents | Nonbonded energy (kcal/mol) |
---|---|
Clay-modifier backbone | −894 |
Clay-modifier functional group | −1176 |
Nonbonded energies of individual atoms of organic modifier with clay.
Component of modifier and clay | Electrostatic energy (kcal/mol) [Col-A] | Van der waals energy (kcal/mol) [Col-B ] | Total nonbonded energy (kcal/mol) [Col-A + Col-B] |
---|---|---|---|
Clay-modifier backbone hydrogen | −2503 | −87 | −2590 |
Clay-modifier backbone carbon | +1954 | −257 | +1697 |
Clay-modifier functional hydrogen | −2392 | −8 | −2400 |
Clay-modifier functional nitrogen | +497 | −0.5 | +497 |
Clay-modifier functional oxygen | +1689 | −48 | +1641 |
Clay-modifier functional carbon | −890 | −25 | −915 |
The nonbonded interaction energies between constituents of PBT-PCN and Nylon6-PCN are presented in Table
Nonbonded energies PCNs.
Component of PCN | Electrostatic energy (kcal/mol) | Van der waals energy (kcal/mol) | Total nonbonded energy (kcal/mol) |
---|---|---|---|
[Col-A] | [Col-B ] | [Col-A + Col-B] | |
PBT-PCN | |||
| |||
Clay-polymer | −36 | −165 | −201 |
Clay-modifier | −1836 | −316 | −2152 |
Polymer-modifier | −93 | −43 | −136 |
| |||
Nylon6-PCN | |||
| |||
Clay-polymer | 12 | −182 | −170 |
Clay-modifier | −1464 | −375 | −1839 |
Polymer-modifier | −209 | −30 | −239 |
Nonbonded energies of modifier and polymer with clay.
Component of modifier, polymer, and clay in PCN | Electrostatic energy (kcal/mol) | Van der waals energy (kcal/mol) | Total nonbonded energy (kcal/mol) |
---|---|---|---|
[Col-A] | [Col-B ] | [Col-A + Col-B] | |
PBT-PCN | |||
| |||
Clay-modifier backbone | −582 | −282 | −864 |
Clay-modifier functional group | −1255 | −34 | −1289 |
Clay-polymer backbone | −250 | −108 | −358 |
Clay-polymer functional group | +213 | −56 | +157 |
| |||
Nylon6-PCN | |||
| |||
Clay-modifier backbone | −485 | −297 | −782 |
Clay-modifier functional group | −979 | −77 | −1056 |
Clay-polymer backbone | −183 | −123 | −306 |
Clay-polymer functional group | +155 | −62 | +93 |
Polymer functional group has negative partial charges as shown in Figure
Nonbonded energies between individual atoms of organic modifier and clay.
Individual atoms of organic modifiers and clay | Electrostatic energy (kcal/mol) | Van der waals energy (kcal/mol) | Total nonbonded energy (kcal/mol) |
[Col-A] | [Col-B ] | [Col-A + Col-B] | |
| |||
PBT-PCN | |||
| |||
Clay-modifier backbone hydrogen | −2512 | −67 | −2579 |
Clay-modifier backbone carbon | +1930 | −215 | +1715 |
Clay-modifier functional hydrogen | −2564 | −7 | −2571 |
Clay-modifier functional nitrogen | +546 | +37 | +583 |
Clay-modifier functional oxygen | +1628 | −42 | +1586 |
Clay-modifier functional carbon | −865 | −22 | −887 |
| |||
Nylon6-PCN | |||
| |||
Clay-modifier backbone hydrogen | −2085 | −72 | −2157 |
Clay-modifier backbone carbon | +1600 | −226 | +1374 |
Clay-modifier functional hydrogen | −2071 | −7 | −2078 |
Clay-modifier functional nitrogen | +444 | −2 | +442 |
Clay-modifier functional oxygen | +1359 | −44 | +1315 |
Clay-modifier functional carbon | −711 | −24 | −735 |
Nonbonded energies between individual atoms of polymer and clay.
Component of polymer and clay in PCN | Electrostatic energy (kcal/mol) [Col-A] | Van der waals energy (kcal/mol) [Col-B ] | Total nonbonded energy (kcal/mol) [Col-A + Col-B] |
---|---|---|---|
PBT-PCN | |||
| |||
Clay-polymer aromatic hydrogen | −301 | −10 | −311 |
Clay-polymer backbone hydrogen | −380 | −10 | −390 |
Clay-polymer aromatic carbon | +188 | −59 | +129 |
Clay-polymer backbone carbon | +243 | −29 | +214 |
Clay-polymer functional hydrogen | −90 | −0.2 | −90.2 |
Clay-polymer functional oxygen | +1127 | −36 | 1091 |
Clay-polymer functional carbon | −824 | −20 | −844 |
| |||
Nylon6-PCN | |||
| |||
Clay-polymer backbone hydrogen | −1056 | −19 | −1075 |
Clay-polymer backbone carbon | +872 | −104 | +768 |
Clay-polymer functional nitrogen | +593 | −27 | +566 |
Clay-polymer functional hydrogen | −424 | −2 | −426 |
Clay-polymer functional oxygen | +589 | −13 | +576 |
Clay-polymer functional carbon | −600 | −20 | −620 |
Clay-polymer backbone hydrogen | −1056 | −19 | −1075 |
Nonbonded energies between polymer and organic modifier.
Component of modifier and polymer in PCN | Electrostatic energy (kcal/mol)[Col-A] | Van der waals energy (kcal/mol)[Col-B ] | Total nonbonded energy (kcal/mol) [Col-A + Col-B] |
---|---|---|---|
PBT-PCN | |||
| |||
Polymer backbone—modifier backbone | +115 | −29 | +86 |
Polymer backbone—modifier functional | +209 | −10 | +199 |
Polymer functional—modifier backbone | −132 | −12 | −144 |
Polymer functional—modifier functional | −285 | +7 | −278 |
| |||
Nylon6-PCN | |||
| |||
Polymer backbone—modifier backbone | +58 | −20 | +38 |
Polymer backbone—modifier functional | +91 | −11 | +80 |
Polymer functional—modifier backbone | −94 | −11 | −105 |
Polymer functional—modifier functional | −264 | +11 | −253 |
As mentioned in Section
Energy map of PBT-PCN system.
Energy map of nylon6-PCN system.
Our previous work has shown that the molecular interactions between clay, modifier, and polymer results in significant alteration of polymer to a very large extent in PCN [
In the current work with PBT-PCN system we have observed reduction in crystallinity and improved mechanical properties of the PCN with respect to the pristine polymer. We have seen similar phenomena of reduction in crystallinity and improved mechanical properties of Nylon6-PCN [
The comparison of interaction energy maps for the two PCNs shown in Figures
A comprehensive assessment of PBT-PCN and Nylon6-PCN systems using molecular dynamics and experimental techniques has been conducted. Experimental results and interaction energy maps of PBT-PCN system indicate that the underlying mechanisms of change in crystallinity and improvement in mechanical properties as proposed by altered phase theory are valid. Although the same amount of modified clay is used in the preparation of PCN with PBT and Nylon6, a dramatic difference in change in crystallinity and improvement in mechanical properties is observed. It appears that a polymer with higher crystallinity could require significantly higher attractive and repulsive interaction energies between the polymer and modifier compared to a polymer with lower crystallinity to achieve similar magnitude of percent change in crystallinity and improvement in mechanical properties of PCN.
The authors acknowledge the support of NSF-EPSCoR FlexEM grant for the work conducted. The authors also acknowledge the computational resources provided by the NDSU Center for high performance computing. The nanoindentation and spectroscopy instruments were obtained from NSF grant #0315513 and 0320657.